Performs the proximal-gradient mapping operation to estimate a regularized version of the inverse cov. matrix. Follows the procedure described in, http://dept.stat.lsa.umich.edu/~yvesa/sto_prox.pdf
prox_gradient_mapping(data, theta_start, update_w, update_change, regularizer,
max_iter, tol)
N x P matrix corresponding to the raw data.
Initial value for precision estimate.
Step size for prox-gradient mapping.
Proportion of update_w
to keep when
the algorithm fails to successfully estimate precision.
Regularizing constant, lambda.
Number of mapping iterations.
Tolerance at which the algorithm stops running.
Theta (precision matrix) estimate.