Add CI using pytest & Github Actions

[mscho527]

We would eventually want the example code in example (and possibly a few more scripts) exist as a pytest routine that gets checked everytime a commit (or PR) is submitted. This issue should track the necessary steps until we have a fully working CI environment set up in the repository.

List originally suggested by @oimeitei

• [ ] 1-electron energy, for each fragments
• [ ] 1-electron energy for the full system, technically building full 1-rdm -> contract with 1e- integral should give identical result to building each fragment at a time.
• [ ] same for two rdm, may be an additional check for only the Kumulant.
• [x] HF energy for the full system
• [ ] one-step density matching
• [x] chemical potential matching.
• [ ] writing & reading Hamiltonian

A few more tasks for CI integration

• [x] Set up Github Actions Workflow
• [ ] Set up codecov

[oimeitei]

Yes, I have been planning to add this. We can set up a separate test with testing for different things like say energy, density, ... Can you start adding some test sets (maybe those from example cases)?