Added the hidden `chm_factorize_p` function

This function, which calls `cholmod_factorize_p`, allows for updating
the factorization for A + beta*speye(size(A)...) without explicitly
forming the sum.  This is convenient and saves a bit of time in some
of my work.
Merge pull request #2927 from dmbates/master

Add a hidden convenience function `chm_factorize_p` to LinAlg.CHOLMOD