Avoid constructing amplitudes from passed arrays multiple times

[Schoyen]

This is related to #39, and seen in relation with #35. A solution is to construct an internal _update_hamiltonian-method which takes in the current time and an amplitudes object (either an AmplitudeContainer or an OACCVector) instead of an array. We can then let update_hamiltonian be the exposed function taking in an array, constructing the amplitudes from the template and passing this in to _update_hamiltonian. This lets __call__ and other functions call _update_hamiltonian directly thus limiting the number of times the amplitudes needs to be constructed from an array.