More logic of the ccv method should be moved to ccv.py

More logic of the [ccv method ](https://github.com/py-econometrics/pyfixest/blob/ef1d0449e5af4b1e697c5edb6d496a1e07d456d5/pyfixest/estimation/feols_.py#L1356)should be moved to `ccv.py`