Reduce memory usage in covariance estimation.

When using the SPARSE_QR algorithm, now a Q-less
factorization is used. This results in significantly
less memory usage.

The inversion of the semi-normal equations is now
threaded using openmp. Indeed if one has SuiteSparse
compiled with TBB, then both the factorization
and the inversion are completely threaded.

Change-Id: Ia07591e48e7958d427ef91ff9e67662f6e982c21