Refactor formula parsing

[leostimpfle]

This is a proof of concept for a refactor of PyFixest's formula parsing. The PR introduces a new module parse that refactors formula parsing from the ground up.

The core logic is implemented in pyfixest.estimation.formula.parse.parse which takes in a formula string and returns a collection of parsed formulas represented by pyfixest.estimation.formula.parse.Formula.

All references to the old FormulaParser are bypassed (mostly by renaming the old FixestFormula using imports of the form from pyfixest.estimation.formula.parse import Formula as FixestFormula)

[codecov]

Codecov Report

❌ Patch coverage is 87.06140% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyfixest/estimation/formula/factor_interaction.py	63.41%	30 Missing ⚠️
pyfixest/estimation/formula/parse.py	92.39%	14 Missing ⚠️
pyfixest/estimation/formula/model_matrix.py	92.92%	8 Missing ⚠️
pyfixest/estimation/formula/utils.py	33.33%	6 Missing ⚠️
pyfixest/estimation/model_matrix_fixest_.py	80.00%	1 Missing ⚠️

Flag	Coverage Δ
core-tests	72.65% <87.06%> (-2.25%)	⬇️
tests-extended	?
tests-vs-r	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
pyfixest/did/did2s.py	89.62% <100.00%> (ø)
pyfixest/errors/__init__.py	100.00% <100.00%> (ø)
pyfixest/estimation/FixestMulti_.py	77.95% <100.00%> (-0.24%)	⬇️
pyfixest/estimation/__init__.py	100.00% <100.00%> (ø)
pyfixest/estimation/fegaussian_.py	86.66% <100.00%> (ø)
pyfixest/estimation/feglm_.py	79.16% <100.00%> (ø)
pyfixest/estimation/feiv_.py	86.79% <100.00%> (ø)
pyfixest/estimation/felogit_.py	88.23% <100.00%> (ø)
pyfixest/estimation/feols_.py	86.80% <100.00%> (-4.59%)	⬇️
pyfixest/estimation/feols_compressed_.py	80.00% <100.00%> (ø)
... and 10 more

... and 7 files with indirect coverage changes

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[s3alfisc]

After first look at the code base: much better and much cleaner than before. No fundamental suggestions for improvement from my side. Thank you!

[leostimpfle]

Note to self: Look into allowing formulaic's multi stage formula notation in the parser. For more background see matthewwardrop/formulaic#108 and matthewwardrop/formulaic#24