Separated convergence parameters in CV

In GroupSparseCovarianceCV, tolerance and maximum iteration number can
have different values for the alpha selection phase, and the final
optimization. Two keywords have been added: tol_cv and max_iter_cv.
Example comparing sparse precision estimation
Removed "assume_centered" keyword in g.s.c.

The "assume_centered" keyword has been removed from every function
related to group-sparse covariance (useless).