Add split and fsplit arguments to feols and fepois

[s3alfisc]

Add split and fsplit argument to feols and fepois.

So far, it is only supported to look through all categories of a split or fsplit value. Addresses #10 .

After merging this PR, we can easily loop over multiple samples by running

%load_ext autoreload
%autoreload 2

import pyfixest as pf

data = pf.get_data()

fit = pf.feols(
    fml = "Y ~ X1 + csw(f3, X2)", 
    data = data, 
    fsplit = "f1"
)

pf.etable(fit)

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyfixest/estimation/FixestMulti_.py	93.22%	4 Missing ⚠️
pyfixest/report/summarize.py	33.33%	2 Missing ⚠️
pyfixest/estimation/feols_.py	87.50%	1 Missing ⚠️
pyfixest/report/visualize.py	50.00%	1 Missing ⚠️

Flag	Coverage Δ
core-tests	77.43% <90.90%> (+0.01%)	⬆️

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

Files with missing lines	Coverage Δ
pyfixest/errors/__init__.py	100.00% <ø> (ø)
pyfixest/estimation/FormulaParser.py	96.50% <100.00%> (+0.10%)	⬆️
pyfixest/estimation/estimation.py	95.65% <100.00%> (+0.53%)	⬆️
pyfixest/estimation/feiv_.py	98.16% <ø> (ø)
pyfixest/estimation/feols_compressed_.py	83.33% <ø> (ø)
pyfixest/estimation/fepois_.py	87.80% <ø> (ø)
pyfixest/estimation/feols_.py	82.92% <87.50%> (+0.02%)	⬆️
pyfixest/report/visualize.py	54.83% <50.00%> (ø)
pyfixest/report/summarize.py	85.97% <33.33%> (-0.33%)	⬇️
pyfixest/estimation/FixestMulti_.py	80.61% <93.22%> (-1.11%)	⬇️