Add type parameter for backend

[SouthEndMusic]

This might clean up the code somewhat, because:

1. It is easily validated at construction that all fields of an object have the same backend, which can now go wrong;
2. The backend can be obtained from the type parameter instead of calling get_backend at the beginning of most methods.

Some investigation might be needed to check whether the backend type (e.g. CPU) or the backend type instance (CPU()) should be the type parameter, as I think I saw that these backends can have some internal state.