Formation energy calculator

[xivh]

• add formation energy calculator
• add a temporary test for one of my binary data sets

Questions

• should the calculator store a CompositionConverter inside of it? right now it requires parametric compositions
• how should I do more extensive testing?

[bpuchala]

Please do PRs to the 2.X branch, not main.

[bpuchala]

@xivh Thanks for this! I ended up merging it, then modifying it. Because the test data wasn't there I made a very simple test. I would be happy to add more test cases, but perhaps we can just hard code a few of the test energies in the test method.