Add own implementation for printing an unknown function

Using the SymPy implementation for printing a sympy.Function has some issues:

• you cannot use cast_func as an argument -> SymPy prints `cast_func(2,float)
• typed numbers are not typed any more -> e.g. pow(x, 0.25) instead of pow(x, 0.25f)

Maybe a disadvantage or an advantage:

• arbitrary Functions: sympy.Function('my_function') are now allowed.

Would have prevented commits like:

Fix: type of sqrt(int) was int not floating point type

pystencils/backends/cbackend.py

@@ -385,7 +385,8 @@ class CustomSympyPrinter(CCodePrinter):
         elif expr.func == int_div:
             return "((%s) / (%s))" % (self._print(expr.args[0]), self._print(expr.args[1]))
         else:
-            return super(CustomSympyPrinter, self)._print_Function(expr)
+            arg_str = ', '.join(self._print(a) for a in expr.args)
+            return f'{expr.name}({arg_str})'
 
     def _typed_number(self, number, dtype):
         res = self._print(number)