Add AvailabilityModel enum for plant availability models

[chris-ashe]

Description

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 70.83333% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.66%. Comparing base (808481e) to head (e720c8e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
process/models/availability.py	75.00%	5 Missing ⚠️
process/core/scan.py	50.00%	1 Missing ⚠️
process/core/solver/objectives.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4468      +/-   ##
==========================================
+ Coverage   48.64%   48.66%   +0.01%     
==========================================
  Files         151      151              
  Lines       29704    29721      +17     
==========================================
+ Hits        14449    14463      +14     
- Misses      15255    15258       +3     

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[timothy-nunn]

I believe you are still missing some places where the switch value is checked:

[timothy-nunn]

Couple of questions about code that seems might be outdated and from when we only had 1 availability model. Otherwise happy with code changes you have made.

[timothy-nunn]

Is there a reason this only applies to i_plant_availability == 1? I find it more likely that this should be checking AvailabilityModel(self.data.costs.i_plant_availability) != AvailabilityModel.INPUT... I.e. don't scan if f_t_plant_available is not being input because the scanned value will get overwritten by the model

[timothy-nunn]

Similar question, is this correct anymore. Shouldn't you be able to optimise if using model 1, 2, or 3?