If you Google "pybind11 subclass" you will find a couple of threads
on how to solve the problem, none of which worked in this scenario.

The issue on the pybind github about subclasses (below) cites a way
to subclass an existing binding that feels as robust as creating
a class on the fly using type("MyClass", bases, attrs)
 -> pybind/pybind11#1193

Another issue recommends using py:base, which has been deprecated.
 -> pybind/pybind11#17

...also it didn't work.

It seems to me that because we are defining each class as an .inl rather
than a .hpp or .cpp file, they are not aware of each other. To remedy
this, I've included the MDGModifier binding in the MDagModifier file,
with an pre-processor to prevent duplicate entries. Otherwise, the base
class would be omitted in the main.cpp.

I think this solution will work well when we get to other classes with
many subclasses, like the whole MFn* tree.