Bugfix/issue 733 function void args

#### Summary:

This request ensures that user-defined functions with void argument types print a warning message and fail to parse. 

#### Intended Effect:

See summary.  This fixes issues #733.

#### How to Verify:

There is a test in test/unit/gm/parsers along with a sample model that will parse under the 2.3 release but fail to parse under this patch.

#### Side Effects:

None. 

#### Documentation:

I updated the language BNF in the manual, and also added some notes that void types may not be used for arguments near the discussion of void returns.

#### Reviewer Suggestions:

Anyone.