Improve Unit Tests

[mtfishman]

We should set up CI for NDTensors.jl.

Unfortunately the tests are not very comprehensive right now. Also, a lot of the functionality is tested through ITensors.jl, and it is important to make sure that NDTensors works with ITensor indices. Possibly we could have NDTensors run ITensors tests (and force it to use the current branch of NDTensors?), as suggested here: #16.

[emstoudenmire]

CI is now set up, but we should still do the following:

1. improve tests to be more comprehensive
2. possibly run ITensors tests with NDTensors testing action