Implement superoperator representation transformation

[labay11]

Problem Description

Implement functions to convert from and to kraus, choi, chi and superoperator.

Proposed Solution

All the functions are already implemented in qutip in https://github.com/qutip/qutip/blob/master/qutip/core/superop_reps.py

Additionally, in metrics we could implement the process fidelity as in https://github.com/qutip/qutip/blob/master/qutip/core/metrics.py#L152

Alternate Solutions

No response

Additional Context

No response

[ytdHuang]

@labay11
We actually announced a previous issue #181 a few months ago.
But we don't have time for developing this functionality so far.
Would you like to contribute ?

[labay11]

Yes, if there's no hurry for implementing this I can try to do it

[ytdHuang]

Okay, no rush~
I suggest that you try it after PR #266 is merged.
Because there's a huge change of the code in that PR.

[ytdHuang]

The features I mentioned in PR #271 might be necessary for this PR (especially about stinespring representation).