Created GroupSparseCovariance

Improved user messages in group_sparse_covariance
Moved LogMixin out of nifti_region.py.
Added duality gap computation

The return_costs keyword of group_sparse_covariance nows returns the
duality gap in addition to the primal problem cost.
Small optimization of cost computation

duality gap and objective in group_sparse_covariance are now computed
inside a single function. This is faster than the previous two-function
implementation.