Bugfix/issue 734 user fun name var conflict

#### Summary:

Disallow user-defined function names as variables.  Fixes issue #734

#### Intended Effect:

See summary.  When it was allowed, it caused problems as outlined in issue #734

#### How to Verify:

The added model test run from test/unit/gm/parser would fail in the 2.3 release and succeeds now with the right error message being thrown.

#### Side Effects:

None.

#### Documentation:

Added a section in the language manual indicating that user-defined function names can't be used as variable names.  

#### Reviewer Suggestions:

Anyone.