proposal for a generic interface to specify various chain Hamiltonians

I have a proposal for a flexible interface to specify chain Hamiltonians.

To this end I wrote some helper functions in model_helper.py that can be reused in any function making a Hamiltonian, and that deals with all the different possible user inputs.

I adapted interacting_kitaev.py to use this interface, and usage_example.py shows some examples.

Curious to hear about your opinion on the interface!