Feature/issue 823 check multi prob autodiff

#### Summary:
Check higher order autodiff for multivariate probability distributions (issue #823). This shouldn't be merged before issue #838. [DL: 838 has been merged]

#### Intended Effect:
Verify that multivariate distributions can handle higher order autodiff.

#### How to Verify:
Run unit tests.

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone