Reopen iterative rectifier PR

[leostimpfle]

Reopens the original PR #534 as discussed with @s3alfisc

[s3alfisc]

We have a very small test error:

    def test_separation():
        """Test separation detection."""
        y = np.array([0, 0, 0, 1, 2, 3])
        df1 = np.array(["a", "a", "b", "b", "b", "c"])
        df2 = np.array(["c", "c", "d", "d", "d", "e"])
        x = np.random.normal(0, 1, 6)
    
        df = pd.DataFrame({"Y": y, "fe1": df1, "fe2": df2, "x": x})
    
        with pytest.warns(
            UserWarning, match="2 observations removed because of separation."
        ):
>           fepois("Y ~ X  | fe1", data=example1, vcov="hetero", separation_check=["fe"])  # noqa: F841
E           NameError: name 'example1' is not defined

tests/test_poisson.py:34: NameError

I think renaming data = example1 to data = df should solve this =)

[s3alfisc]

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[s3alfisc]

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[s3alfisc]

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[codecov]

Codecov Report

Attention: Patch coverage is 89.55224% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
pyfixest/estimation/fepois_.py	88.52%	7 Missing ⚠️

Flag	Coverage Δ
core-tests	77.62% <89.55%> (+0.15%)	⬆️

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

Files with missing lines	Coverage Δ
pyfixest/estimation/FixestMulti_.py	80.71% <100.00%> (+0.09%)	⬆️
pyfixest/estimation/estimation.py	95.87% <100.00%> (+0.22%)	⬆️
pyfixest/estimation/fepois_.py	87.55% <88.52%> (-0.25%)	⬇️

[s3alfisc]

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[s3alfisc]

Added minor changes. I think this is good to merge now! Thanks Leo @leostimpfle =)