add custom swig typemap for C++ enums that does value checks

[stevengj]

e.g. when passing enums like direction or boundary_side to functions like fields::set_boundary, right now SWIG just does static_cast from an integer value (int), which performs no checks and could result in a crash if the user passes something invalid.

A simple solution would be to just add SWIG typemaps for these enum types that checks whether the value is in range and otherwise throws ValueError or something.