Handling time independent Hamiltonians

[brownadder]

Currently, all Hamiltonians must be specified as a f(t), Hop pair. For time-independent Hamiltonian technically one can achieve this by passing a function f such that f(t) =1 for all t. However, this becomes hard to detect in code and we cannot exploit time-independence of a Hamiltonian.

We need a way to specify a time independent Hamiltonian without passing f(t)

[brownadder]

We should also allow 2 sigma_x to be expressed easily, i.e. real-valued scalar time-independent multiples

[brownadder]

Maybe we express scalar multiples as (2, Hop(N,k,sigma_x))?