Document, Test, Repair `gempyor.subpopulation_structure` Module

[TimothyWillard]

Describe the bug

The gempyor.subpopulation_structure module has some documentation, but it is lacking and could use some touching up, and the tests are currently in tests/seir/test_subpopulationstructure.py and could use consolidation and cleaning up. Will also want to "repair" the module as issues are discovered (remove dead code, consolidate duplicated code, move TODO comments to GitHub issues, etc.). Similar to GH-246.

To Reproduce

N/A

Expected behavior

• Clearly documented gempyor.subpopulation_structure module, including module documentation and SubpopulationStructure class documentation.
• Unit tests contained within tests/subpopulation_structure/ that provide coverage for the exceptions produced by the SubpopulationStructure class and validate its attributes with successful instantiation.
• Either "repair" the module in this issue if the changes are small or create further GitHub issues as needed if the changes are substantial.

Environment, if relevant:

• flepiMoP branch and/or commit if relevant (git rev-parse HEAD): main, b05e92e16501fd6c3fb0d1ac1c374b578b0ddbc7