Add factory module for extra misfit terms

[keurfonluu]

• Added: a new submodule evodcinv.factory to group commonly used extra misfit terms (e.g., regularization) (see Output models  #15).
• Changed: no longer normalize misfit. This change is to get a "physical" meaning out of RMSE misfit when there are no input uncertainties (e.g., RMSE can be interpreted as average misfit in km/s when inverting dispersion curves). This change should slightly affect previous simulations (i.e., with the same seed).

Reminders:

• Run invoke format to make sure the code follows the style guide,
• Add tests for new features or tests that would have caught the bug that you're fixing,
• Write detailed docstrings for all functions, classes and/or methods,
• If adding new functionality, unit test it and add it to the documentation.

[codecov]

Codecov Report

Patch coverage: 51.72% and project coverage change: -0.33% ⚠️

Comparison is base (a290e72) 53.03% compared to head (e490fe2) 52.70%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   53.03%   52.70%   -0.33%     
==========================================
  Files          17       19       +2     
  Lines         643      666      +23     
==========================================
+ Hits          341      351      +10     
- Misses        302      315      +13     

Files Changed	Coverage Δ
evodcinv/_progress.py	100.00% <ø> (ø)
evodcinv/_result.py	20.10% <0.00%> (-0.11%)	⬇️
tests/test_model.py	100.00% <ø> (ø)
evodcinv/factory/_extra_terms.py	38.88% <38.88%> (ø)
evodcinv/_curve.py	56.33% <50.00%> (-0.19%)	⬇️
evodcinv/__init__.py	100.00% <100.00%> (ø)
evodcinv/_model.py	74.25% <100.00%> (-0.13%)	⬇️
evodcinv/factory/__init__.py	100.00% <100.00%> (ø)

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