Probing of group_sparse_covariance
Probing of group_sparse_covariance

Added keyword "probe_function" to group_sparse_covariance. This is a
way to provide a function that is called after each iteration. See
aspect-oriented programming.

Updated the standard message displayed for each iteration, to show the
max norm of the difference between two estimation of the precision
matrices.

abenicho/isvr