Add _ext_module variable to base python classes

[astatt]

Description

Without the variable _ext_module some of the base python classes cannot be easily extended in external plugins, because it will not correctly find it, i.e when wanting to use or import "plugin_name.wall.PotentialName" it will instead look for "hoomd.wall.PotentialName". The simple inclusion of "_ext_module = _md" that can be overwritten by the plugin fixes this issue.

Motivation and context

Needed for convenient development of external plugins.
Resolves Issue #1852.

How has this been tested?

This has been tested locally on the CPU. Because it only touches *.py files, no other tests are necessary.

Change log

Fix: External walls, external fields, and constrains can now be implemented via external plugins 

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.

[joaander]

Thanks!