Add Python get_include() (#141)

Introduces a mscclpp.get_include() in the Python module.
The extension module is now named _mscclpp so that we can have
Python code in the mscclpp module.
Also does some miscellaneous cleanup.

python/mscclpp/__init__.py

@@ -0,0 +1,10 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT license.
+
+from ._mscclpp import *
+import os as _os
+
+
+def get_include():
+    """Return the directory that contains the MSCCL++ headers."""
+    return _os.path.join(_os.path.dirname(__file__), "include")