Make DagModifier a subclass of DGModifier

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.

src/MDGModifier.inl

@@ -1,3 +1,5 @@
+#ifndef DGMODIFIER_INL
+#define DGMODIFIER_INL
 py::class_<MDGModifier>(m, "DGModifier")
     .def(py::init<>())
 
@@ -518,4 +520,5 @@ This requirement is used when the plugin is checked to see if it is in use, or i
 For compound attributes only the topmost parent attribute may be passed in and all of its children will be unlinked, recursively. 
 Thus it's not possible to unlink a child attribute from a plugin by itself. 
 
-Note that the link is broken immediately and is not affected by the modifier's doIt() or undoIt() methods.)pbdoc");
+Note that the link is broken immediately and is not affected by the modifier's doIt() or undoIt() methods.)pbdoc");
+#endif // DGMODIFIER_INL

src/MDagModifier.inl

@@ -1,4 +1,6 @@
-py::class_<MDagModifier>(m, "DagModifier", "DGModifier")
+#include "MDGModifier.inl"
+
+py::class_<MDagModifier, MDGModifier>(m, "DagModifier")
     .def(py::init<>())
 
     .def("createNode", [](MDagModifier & self, std::string type, MObject parent = MObject::kNullObj) -> MObject {

tests/test_MDagModifier.py

@@ -9,9 +9,6 @@
 from . import assert_equals, as_obj, as_plug, new_scene
 
 def test_createNode():
-    raise SkipTest("Cannot test DAGModifier.createNode - not sure how to subclass from DGModifier.")
-
-
     node = as_obj(cmds.createNode('transform'))
     type_id = cmdc.FnDependencyNode(node).typeId()