Refactor GA Output Directory Code into Reusable Base Class

[electronsandstuff]

Note This is stacked on top of #454. Wait until merged to handle.

This PR refactors the NSGA2 output code into a reusable base class to enable the addition of other genetic algorithms without duplicate code.

• New base class GAGeneratorBase that includes functionality for checkpointing, population, and data output.
• Refactor NSGA2Generator to use GAGeneratorBase for output, simplifying the class.
• New unit tests for output functionality.
• output_dir now expands environment variables and home character (~)
• output_dir accepts any os.PathLike
• Add vocs.txt output back in. I removed it from Refactor Generator Checkpointing into Reusable Class #454 as VOCS data is no longer required to be stored separately to load the serialized generator from a checkpoint. However, I realized that this data is still needed for analysis of optimization results.

TODO

• Expand home and env variables in output_dir
• Accept any os.PathLike
• Add VOCS output back to the output directory. This is necessary for analysis of data after the fact without the config file present.

[codecov]

Codecov Report

❌ Patch coverage is 98.22485% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
xopt/generators/ga/base.py	93.05%	2 Missing and 3 partials ⚠️
xopt/tests/generators/ga/test_base.py	99.29%	1 Missing ⚠️

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