Feature/issue 800 check positive finite

#### Summary:
Add check_positive_finite (issue #800) to consolidate some of the error checking. I didn't actually run the distribution tests, but I did run the unit tests and test-headers, both of which passed.

#### Intended Effect:
Add check_positive_finite (issue #800) to consolidate some of the error checking.

#### How to Verify:
Run unit tests and distribution tests.

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone.