Add the ability to load Pauli strings

[BrendanKKrueger]

If we're going to claim it in the paper, we should really add it. And it should be easy to quickly add at least a very basic version of this.

Reuben is working on grouping, and he may end up modifying this and/or creating a second reader that does something similar but supports grouping.

• Add testing

[BrendanKKrueger]

There are now three ways to transfer a Hamiltonian from hamgen.py to qre_driver.py: the tensors file, the Pauli strings file (dense strings), and the Pauli strings file (sparse strings). The two Pauli strings files are consistent with each other, but the tensors file is a little different. I'm not sure how big of a concern that is -- how consistent should the results be?

format	Clifford	T	qubit
dense Pauli	9985041685	5343554322	23
sparse Pauli	9985041685	5343554322	23
tensors	9978292301	5337820622	23