[ENH/WIP] - Add general support for parameter conversions

[TomDonoghue]

This generalizes the procedure for doing post-fitting parameter conversions.

To do so, it introduces "ParameterConverters". For any parameter with a requested / provided converter, this conversion gets called on the fit parameter value after the model fitting process.

In doing so, this update removes parameter conversion from the Algorithm, and makes it a separate process. Note that in terms of default behavior, this PR does not change anything (the same set of post-fitting parameter conversions are defined are previous versions of specparam 2.0 & fooof 1.X, and they are accessed / returned in the same way).

Notes:

• This might not be the final approach / organization for managing parameter conversions
  • ToDo: figure this out more (e.g. might store as part of Modes, and need to consider loading definitions)

[codecov]

Codecov Report

❌ Patch coverage is 99.36306% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.61%. Comparing base (f2f771b) to head (1e8b05c).

Files with missing lines	Patch %	Lines
specparam/models/model.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #383      +/-   ##
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+ Coverage   97.56%   97.61%   +0.05%     
==========================================
  Files         152      160       +8     
  Lines        6542     6680     +138     
==========================================
+ Hits         6383     6521     +138     
  Misses        159      159              

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