Function meta objects now also include information about the
corresponding sympy function, used in parse_to_sympy to not treat exp,
sin, etc. as undefined, arbitrary functions. This might allow for
simplifications in the code. brian.utils.parsing had to move to
brian2.codegen to make this work because it depends on
brian2.codegen.functions.numpyfunctions now.
Assume that symbols are real values (and not complex) -- this allows
sympy to replace expressions like sqrt(x**2) by abs(x), for example.