updated multinomial, fixes #499

[from Bob's original pull request: #501. Recreated to merge back into develop instead of hotfix/v2.1.1]

#### Summary:

Fixes issue 499 with wrongly sized multinomial_rng return; bonus fix of arithmetic error overflowing binomial

#### Intended Effect:

See above.

#### How to Verify:

Run tests.

#### Side Effects:

None.

#### Documentation:

The doc's correct.

#### Reviewer Suggestions:

Anyone.