GPU backend for QuTip

QuTiP's data layer provides the mathematical operations needed to work with quantum states and operators, i.e. Qobjs,. As part of Google Summer of Code 2020, the data layer was rewritten to allow new backends to be added more easily. Backends using in-memory spares and dense matrices already exist, and we would like to add a backend that runs on GPU, either through Cupy or TensorFlow. This will decrease the computation time thus making it feasible to work on larger qubit systems.