Add a contextmanager and reverse default option

[Ericgig]

The set_as_default as a new parameter: reverse which restore qutip's core default to undo the operation.

The backend can also be used with context:

>>> with qutip_cuquantum.CuQuantumBackend(cudm_ctx):
>>>     in_ctx = qutip.num(4)
>>> 
>>> out_ctx = qutip.num(4)
>>> 
>>> print(in_ctx.dtype, out_ctx.dtype)
<class 'qutip_cuquantum.operator.CuOperator'> <class 'qutip.core.data.dia.Dia'>