Add Qobj.get to obtain the data in common type

[Ericgig]

Description
I think that with the different data type a Qobj can contain, it would be useful to be able get the underlying data (ndarray, csr_matrix, jax.Array, etc.) easily from the Qobj. I added the method Qobj.get(copy) for that.

I allow direct access to the object in case it's needed for auto-differentiation or other advanced features.

I am open to a better name than get if it's too generic.

[coveralls]

Coverage: 75.371% (+0.05%) from 75.317% when pulling fcd936e on Ericgig:feature.get into df08d96 on qutip:master.

[hodgestar]

I like this new interface and I'm fairly happy with the name extract unless someone thinks of a better option.

The doc strings also need some updating.

[AGaliciaMartinez]

What about data_as, get_as or extract_as? I think adding as to any of the method names could facilitate understanding what this method does without reading documentation. I personally prefer data_as given that data is already an attrinute for qobj.

[Ericgig]

data_as feels more natural than extract... renamed.

[Ericgig]

@hodgestar, is the current naming fine for you? I

[hodgestar]

Sorry, I think I missed the last commit. Happy with the new name.