Add optional ordering of fixed effects

[leostimpfle]

Add feorder argument for optional ordering of fixed effects mirroring the order argument for coefficients. For example:

import pyfixest as pf
import maketables as mt

data = pf.get_data(N=1_000, seed=0, model="
fit = pf.fepois(fml="Y ~ X1 | f1+f2", data=data)
mt.ETable(fit, feorder=["f2", "f1"])