Fix custom function handling in sympy solve visitor

[JCGoran]

When running NMODL with NEURON codegen on this file:
https://github.com/neuronsimulator/reduced_dentate/blob/master/mechanisms/tca.mod

we get the following kind of error, but the codegen proceeds:

[NMODL] [info] :: Running sympy solve visitor
[NMODL] [warning] :: SympySolverVisitor :: python exception. (--verbose=info)
[NMODL] [info] :: Traceback (most recent call last):
# a lot of stack trace here...
sympy.printing.codeprinter.PrintMethodNotImplementedError: Unsupported by <class 'sympy.printing.c.C99CodePrinter'>: minf
Set the printer option 'strict' to False in order to generate partially printed code.

The problem is that functions defined in mod files are not known to the code generator, so it's complaining it cannot do the codegen for them. A somewhat minimal example of this is the following mod file:

UNITS {(mV) = (millivolt)}
NEURON {SUFFIX test_custom_functions}
STATE {m}
ASSIGNED {v (mV)}
INITIAL {m = minf(v)}
BREAKPOINT {SOLVE states METHOD cnexp}
DERIVATIVE states {m' = minf(v)}
FUNCTION minf(v(mV)) {minf = v}