Issue991

[leostimpfle]

Closes #991

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
core-tests	75.81% <100.00%> (?)
tests-extended	?
tests-vs-r	16.14% <0.00%> (+<0.01%)	⬆️

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Files with missing lines	Coverage Δ
pyfixest/estimation/FormulaParser.py	96.46% <100.00%> (+29.63%)	⬆️
pyfixest/estimation/model_matrix_fixest_.py	91.00% <100.00%> (+10.00%)	⬆️

... and 41 files with indirect coverage changes

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[leostimpfle]

I should clarify that this PR allows using spaces in variable names. For example, the following works

import pyfixest as pf
import polars as pl

data = pl.DataFrame({"Y": [1, 2, 3], "cat": ["cat 1", "cat 2", "cat 2"]}).cast(
    {"cat": pl.Categorical}
)

pf.feols("Y ~ i(cat, ref='cat 2')", data=data)

This is achieved by updating _deparse_fml to preserve spaces and a few tweaks around FormulaParser which essentially make formula parsing whitespace aware (by using re.split instead of str.split).