Bugfix/issue 1171 fn signature tests log mix

#### Summary:

Addresses issue #1171 

#### Intended Effect:

Adds 6 function signatures for all permutations of var and double arguments to log_mix 

#### How to Verify:

src/test/unit/gm/parser/math_functions_test.cpp 

#### Side Effects:

None.

#### Documentation:

None

#### Reviewer Suggestions:

Anyone