Add black box tests

[mec07]

This is probably the most important issue. PyLATO is missing basic automated validity tests.

Here are some ideas of some test cases (of course there are so many possibilities):

• test cases that are already in PLATO
• collinear tight binding calculations of two atoms
• collinear tight binding calculations of an infinite line of atoms
• collinear tight binding calculations of two atoms in PBCs
• noncollinear tight binding calculation of two atoms (should be the same as the collinear)

My personal preference would be to use the python behave (https://behave.readthedocs.io/en/latest/) testing framework; however if someone else were to help with this, e.g. @horsfielda, then of course they could choose how to do this.

The .circleci/config.yml file should be updated to run these tests while calculating the coverage, then combine the coverage with the unit tests coverage and then push the combined coverage to codecov.io.

[mec07]

Interestingly enough it looks like I started on this a long time ago, but never got very far. The branch where I started was: add_behave_tests. I've just converted it into a PR: #5

[mec07]

I've merged PR #5 now. The next step is to just add a bunch of good tests. I'd really like to see the test coverage of PyLATO go above 75%.