Add Class Inheritance Diagram to the docs

[PAHdb]

Add Class Inheritance Diagram via :automodapi: -- see https://www.google.com/url?q=https://www.sphinx-doc.org/en/master/usage/extensions/inheritance.html

[mattjshannon]

For example, see figure at bottom of this page: https://pahfit.readthedocs.io/en/latest/?badge=latest

Few options to do this:

• https://github.com/astropy/sphinx-automodapi
• https://www.sphinx-doc.org/en/master/usage/extensions/inheritance.html

[PAHdb]

Diagram added, closing issue.