Setting `qutip-qip` as an optional package for qutip-5

Problem Description

From version 5.0, the subpackage qutip.qip will be removed and replaced by a separate package qutip-qip. To make it easier for others to migrate from 4.7 to 5.0, we can set up qutip-qip as an optional plug-in for qutip, so that if qutip-qip is installed, the qutip.qip name space can be used as before.

Expected behaviour

• If only qutip is installed, importing qutip.qip will raise an ImportError with instruction for installing qutip_qip.
• If qutip-qip is installed, using qutip.qip will automatically use the qutip_qip namespace, with no difference in the user interface.
• Update setup.cfg so that pip install qutip[qip] will automatically install qutip_qip.

E.g. the following should work if qutip-5.0 and qutip-qip are both installed

# The following should be the same
from qutip.qip.circuit import QubitCircuit
from qutip_qip.circuit import QubitCircuit

Proposed Solution

See https://github.com/qutip/qutip/discussions/1500 for ideals on the implementation.

Note: The implementation should be done against the dev.major branch of qutip.

[hodgestar]

Does this entail any extra steps not encompassed in the contributing guidelines, or is it just a matter of branching from and PRing to dev.major instead of master?

Just branching from and PRing to dev.major – the contribution guidelines are the same.

[HGSilveri]

Hey! I’d like to help you out with this, is that okay @nathanshammah @hodgestar ? If there’s a problem with me being a maintainer at Unitaryhack just let me know, no hard feelings of course