Move logic in the fixef and predict method to standalone scripts fixef.py and prediction.py.

Move logic in the [fixef and predict ](https://github.com/py-econometrics/pyfixest/blob/ef1d0449e5af4b1e697c5edb6d496a1e07d456d5/pyfixest/estimation/feols_.py#L1727) method to standalone scripts `fixef.py` and [prediction.py](https://github.com/py-econometrics/pyfixest/blob/master/pyfixest/estimation/prediction.py).