Organize tests; GitHub Action

[zmoon]

In case you are interested, I thought it could be cool to have a test badge as well.

Summary:

• make it easier to run pytest by putting the tests in a folder and including a requirements list
• GH Actions Workflow that runs the tests for a set of Python versions (currently only 2)
  • had to modify the loading from refractiveindex.info in notebook 01 to load it twice (fails first time, happened to me locally as well)
• add badge (but need to correct the URLs for real repo❗)
• correct Au -> Ag in notebook 02

[zmoon]

@scottprahl I am glad I could help. Thanks for all your nice material on adding-doubling and providing this Mie package.

Note that the readme badge link (target and image) still has to be updated to use correct repo (it is still using on my fork, that's why it currently says failing when it should be passing). Just replace zmoon with scottprahl in this part:

.. image:: https://github.com/zmoon/miepython/actions/workflows/test.yml/badge.svg
   :target: https://github.com/zmoon/miepython/actions/workflows/test.yml

[scottprahl]

@zmoon Thanks again. Badge is now fixed.