Implement a code-to-code pdf evaluation verification for GPMSA

[dmcdougall]

[dmcdougall]

For the multivariate case we need the DKRidge nugget effect to be 1e-4. I can't remember what the queso input file option is called.

[dmcdougall]

Clarification: the default is 1e-4, for the verification we need 1e-6. Sorry for the confusion.

[dmcdougall]

Scalar case is done in #614 and #615.