Improve NCH with a better strategy on samples selection

[toncho11]

Current strategy: Using random samples for the hulls.

Closest n matrices : For each class we calculate the closest n (n=10) matrices to the test sample. This will be done using distance_logeuclid() from pyriemann. Next we use these n + n matrices to calculate again 2 distances using qdistance_logeuclid_to_convex_hull(). The shorter distance predicts the class.