in latex: don't use operatorname for single letter functions; properly render functions without arguments
fix quantum printing tests
fix docstring
refactor logic for printing functions in latex. Add logic for printing
user-defined functions without arguments. This does not work for
functions built in to SymPy (e.g., sin, cos, gamma).

Non-greeks could be easily added with a method _print_FunctionClass.
This would not do the right thing for the gamma function, though.
tests for latex changes