Mimic `fixest::i()` by relying on formulaic stateful transforms

[s3alfisc]

Prompted by matthewwardrop/formulaic#238, implement a proper i() operator and replace the ugly string parsing.

Easily done via formulaic stateful transforms:

import numpy as np
import pandas as pd
from formulaic.transforms import stateful_transform
from formulaic.transforms.contrasts import C, TreatmentContrasts
from formulaic import model_matrix
import pyfixest as pf

data = pf.get_data()

@stateful_transform
def i(factor_var, ref=None, _state=None, _metadata=None, _spec=None):

    if "i" not in _state:
        _state["i"] = C(data = factor_var, contrasts = TreatmentContrasts(ref))

    return _state["i"]

model_matrix("i(f1, ref = 1.0)", data = data).head()

Challenge: How to add a second variable that can be interacted with factor_var?

I.e. API as

@stateful_transform
def i(factor_var, var = None, ref=None, ref2 = None, _state=None, _metadata=None, _spec=None):

    if var is None: 
        if "i" not in _state:
            _state["i"] = Formula(C(data = factor_var, contrasts = TreatmentContrasts(ref)))
    else: 
        if "i" not in _state:
            # this does not work, need to find where : interaction implemented in formulaic
            _state["i"] = C(data = factor_var, contrasts = TreatmentContrasts(ref)) : C(data = var, contrasts = TreatmentContrasts(ref2))

    return _state["i"]

Maybe too ambitions (but certainly useful for pooling DiD time periods, i.e. months to years): bin and bin2 arguments to combine multiple fixed effects levels.

[leostimpfle]

I've started to look into this and can see a few ways to solve this using formulaic without excessive string parsing, see matthewwardrop/formulaic#244 (comment)