Fixed incorrect behaviour of REGRESSION when multiple dependent variables are entered

Fixed incorrect behaviour of REGRESSION when multiple dependent variables are entered

The REGRESSION command behaved badly when more than one dependent variable was entered.
The cause was that the unnormalised covariance matrix returned from the covariance
module was mutated for each variable.  Consequently, each variable after the first
was wrong.  This change fixes that by changing the ownership semantics of the returned
matrix (and thereby its constness).

This change also adds some comments, attends to constness and fixes the remaining
memory leaks associated with regression.