NF: One/two-sided tests in permuted OLS (replace F-test with t-test)

Replacement of the permuted F-test with a t-test to afford one-sided tests.
Two-sided tests equivalent to F-tests can still be performed (setting the `two_sided_test` argument to `True`). The example shows how to perform a one-sided test and the opposite test with one single call the permuted OLS (thus reducing the computation costs by two).

The unitary tests are a bit more tricky due to the absence of a simple function performing a massively univariate t_test in the presence of covariates.