Replace aliasing in Feols methods

[s3alfisc]

In almost all methods of Feols, attributes are accessed via aliases. This leads to more lines of code than required, at no gain in readability.

Example: for the _vcov_iid method, we currently have

def _vcov_iid(self):
    N = self._N
    u_hat = self._u_hat
    bread = self._bread
    sigma2 = np.sum(u_hat.flatten() ** 2) / (N - 1)
    vcov = bread * sigma2

    return vcov

but we want

def _vcov_iid(self):
    sigma2 = np.sum(self._u_hat.flatten() ** 2) / (self._N - 1)
    return self._bread * sigma

This applies to practically all the methods of Feols:

• _set_nobs()
• get_fit()
• vcov()
• _vcov_iid
• _vcov_hetero
• _vcov_crv1
• _vcov_crv3_fast
• _vcov_crv3_slow
• get_inference
• wald_test
• wildboottest
• fixef
• get_performance
• ritest