Nonconformity scores for multivariate regression

[M-Mouhcine]

Add nonconfomity score and prediction set functions for conformal multivariate regression in their respective modules.

[josebadalmau]

I have added the difference score function. As for the prediction set functions, the current ones generalize properly to multivariate regression, provided that the features are contained in the last dimension of the arrays. This should be made explicit in the documentation later on.