Unit testing of materials package

[joelvbernier]

Given the critical functionality of the materials package, it is time that we add some basic unit tests to help prevent bad bugs from being merged into master. For starters I suggest the following:

• for the reference materials, compare the data in the HDF5 to what gets instantiated in moth the unitcell and PlaneData classes, including
• cross-check of lattice parameters
• cross check of, say, the first 10 hkls (probably need some additional static data like and npz in material.testing for this)
• check for consistency when switching the default material to a different space group (for each of the other 6 non-cubic lattice types)

[psavery]

If we add a script to the tests directory, it should be automatically included when github actions performs the testing. I can help you guys set that up if you'd like.

[psavery]

So, if I'm recalling things correctly, pytest will import the scripts in that directory and automatically look for functions that start with test_. It assumes those are test functions, and it tries to run them.

There are some other things such as pytest fixtures that are used to make things easier.