ENH: Add Massively Univariate Linear Model with permuted OLS.

This PR provides massively univariate linear models estimation using OLS regression and permutation testing. The code is designed to complete a very large number of permutations (> 100000) on a large problem (thousands of targets variables, namely the brain voxels) within a few minutes. It uses parallel computing and an especially-designed data structure to store the numerous permutations scores.

An example is still needed and more tests may be welcomed.