Implement DynamicsBackend.from_backend

[DanPuzzuoli]

Summary

Closes #53.

This PR is a resubmission of the out-of-date #71 . Its scope is also slightly being expanded to implement the DynamicsBackend.from_backend method, for constructing a DynamicsBackend instance from an existing backend.

Details and comments

This PR is composed of:

• The main bulk (in terms of lines of code) is bringing over the hamiltonian string parser from the Aer PulseSimulator. A single function which wraps this code is introduced in this PR - I'm leaning towards making this an exposed function so that people can potentially use it outside of the from_backend method, though it won't be heavily advertised or developed beyond its existing form.
• The DynamicsBackend.from_backend method, which parses the hamiltonian string of an existing backend, and also imports other backend properties into a DynamicsBackend instance.

[DanPuzzuoli]

I messed this PR up due to some force pushing, it's reopened in #157