Feature/issue 867 more fun args

#### Summary:

Extends the number of arguments allowed in user-defined functions.

#### Intended Effect:

See summary.

#### How to Verify:

Unit tests or you can try your own models.

#### Side Effects:

No.

#### Documentation:

Yes, it's user facing, but it fixes a bug, so the doc doesn't need to be fixed. 

There will still be a platform-specific limit to the number of function arguments.

#### Reviewer Suggestions:

Anyone.