Feature/issue 867 more fun args

#### Summary:

Extend the number of function arguments allowed in user-defined functions beyond 5.

#### Intended Effect:

Extend the number of allowable function arguments in user-defined functions beyond the limit imposed by a flat usage of `boost::promote_args`

#### How to Verify:

I added models with the various arguments in `test-models/syntax-only` starting with `fun_args_`

I added unit tests in `unit/gm/generator_test.cpp` for all of the boundary conditions in the algorithm.  

#### Side Effects:

No. 

#### Documentation:

Behavior now matches existing function doc, which does not prescribe a limit on the number of arguments.

#### Reviewer Suggestions:

Anyone.