fix reference counting for Python function objects

[stevengj]

When Python function objects are passed to Meep, we really need to properly increment/decrement the reference count, e.g. here.

This may mean having some pointer-wrapper class that has virtual constructors, copy constructors, etcetera, that we can use in place of void *user_data pointers, and overload in the Python interface to use incref/decref.

[stevengj]

(Alternatively, we could use C++ closures everywhere, but that is a more invasive change.)