Adding chain.get_daughter_products

[shimwell]

Summary

Add Chain.get_decay_daughters() method that returns all radioactive nuclides reachable via decay from a given parent nuclide.

Details

Unlike Chain.reduce(), this method follows only decay paths (not reactions like neutron capture) and returns a set of nuclide names rather than a new Chain.
Stable nuclides are excluded from the result. Cyclic decay chains are handled without infinite loops.

chain = openmc.deplete.Chain.from_xml('chain.xml')
chain.get_decay_daughters('I135')  # {'Xe135'}                                                                                                                          
chain.get_decay_daughters('Xe135')  # set() (Cs135 is stable)                                                                                                           

Fixes # (issue)

Checklist

• I have performed a self-review of my own code
  - [ ] I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Perhaps it would be better to raise a ValueError if a nuclide is passed that is not in the chain. Do you agree?