Adding strahlenschutzverordnung waste limits

[shimwell]

Summary

• Add German Strahlenschutzverordnung (StrlSchV) 2018 Anlage 4 clearance limits as predefined options
  for Material.waste_disposal_rating()
• Support 8 clearance pathways covering unrestricted release, landfill disposal, incineration, metal recycling, soil, and building rubble
• Clearance values are compared in Bq/g units (German convention), extending the existing Ci/m³ support for NRC/Fetter limits
• When a depletion chain is provided, daughter nuclides already accounted for by a parent's '+' clearance value are excluded from the sum-of-fractions to avoid
  double-counting

Details

The German radiation protection regulation (StrlSchV 2018, Anlage 4, Tabelle 1) defines nuclide-specific clearance values for releasing radioactive materials from
regulatory control. A material passes clearance when the sum-of-fractions (activity/limit for each nuclide) is ≤ 1.0 — the same methodology already used by
waste_disposal_rating() for NRC and Fetter limits.

This PR adds 8 predefined limit sets, each corresponding to a column in Anlage 4:

Limit name	Pathway	Column
StrlSchV_unrestricted	Unrestricted clearance of solid/liquid substances	3
StrlSchV_metal_recycling	Metal scrap recycling	14
StrlSchV_landfill_100	Landfill disposal, facility receiving ≤100 Mg/a	8
StrlSchV_landfill_1000	Landfill disposal, facility receiving ≤1000 Mg/a	10
StrlSchV_incineration_100	Incineration, facility receiving ≤100 Mg/a	9
StrlSchV_incineration_1000	Incineration, facility receiving ≤1000 Mg/a	11
StrlSchV_soil	Soil surface clearance	7
StrlSchV_rubble	Building rubble clearance (>1000 Mg/a)	6

Usage:

rating = mat.waste_disposal_rating(limits='StrlSchV_unrestricted')                                                                                                      
breakdown = mat.waste_disposal_rating(limits='StrlSchV_metal_recycling', by_nuclide=True)                                                                               
 
Handling of '+' (daughter equilibrium) entries                                                                                                                          
                                                          
Some nuclides in Anlage 4 are listed with a '+' suffix (e.g., Cs-137+, Sr-90+), indicating that the clearance value accounts for daughter nuclides in secular           
equilibrium. This convention originates from the Euratom Basic Safety Standards (Council Directive 2013/59/Euratom, Annex VII), which the German StrlSchV implements.
The Euratom directive uses footnote markers rather than a '+' suffix but applies the same principle — the parent's clearance value already includes the dose            
contribution from equilibrium daughters.                  

Of the 91 '+'-only parents in the table, 47 have daughters that also appear as independent entries, which can lead to double-counting in the sum-of-fractions.          
 
An optional chain parameter allows providing a depletion chain to resolve this. When provided, the code walks the decay chain from each '+' parent using                
Nuclide.decay_modes and excludes all daughters whose half-life is shorter than the parent's (i.e., those in secular equilibrium):
                                                                                                                                                                        
from openmc.deplete import Chain                          
chain = Chain.from_xml('chain_endf_b8.0.xml')                                                                                                                           
rating = mat.waste_disposal_rating(limits='StrlSchV_unrestricted', chain=chain)                                                                                         
                                                                                                                                                                        
Without a chain, the calculation still works but may slightly overestimate the rating for materials containing both a '+' parent and its daughters.                     
                                                          
The clearance values were extracted from the official German legal database (https://www.gesetze-im-internet.de/strlschv_2018/anlage_4.html). A parser script           
(parse_strlschv.py) is attached to this PR for reproducibility and future updates to the regulation.
                  

Fixes # (issue)

# Checklist

- [x] I have performed a self-review of my own code
~- [ ] I have run [clang-format](https://docs.openmc.org/en/latest/devguide/styleguide.html#automatic-formatting) (version 18) on any C++ source files (if applicable)~
- [x] I have followed the [style guidelines](https://docs.openmc.org/en/latest/devguide/styleguide.html#python) for Python source files (if applicable)
- [x] I have made corresponding changes to the documentation (if applicable)
- [x] I have added tests that prove my fix is effective or that my feature works (if applicable)

[shimwell]

Converting to draft as I think some more chain functions would be helpful before proceeding with this. So i shall make another PR for those