Upgrade to Qutip 5

[HGSilveri]

EDIT: On standby for eventual modification to the internals of QutipEmulator - for now, Qutip 5 is substantially worse than Qutip 4.
The changes needed to go to Qutip 5 have been pushed to this branch

[HGSilveri]

I gave this a shot and the conclusions were underwhelming (to say the least):

• Qobj is no longer ArrayLike, so we have to explicitly call full() to use numpy ufuncs and sometimes go back to Qobj
• Some methods (eg Qobj.extract_states(), Qobj.eliminate_states()) were deleted without a substitute, so we have to implement them manually
• Execution became dramatically slower: On my laptop, test_simulation.py ran in 8.55s on qutip==4.7.6, now doesn't even complete in 10 minutes! (I gave up and interrupted it)

It's clear that Qutip 5 introduced changes that are incompatible with our current implementation in pulser-simulation. Until that changes, we should stick with Qutip 4.

[awennersteen]

Re: performance there appears to have been a few other reports of this that they were able to fix, but in some cases they're pretty low level

qutip/qutip#2443
qutip/qutip#2406

However, given #757 we might not be able to postpone this forever.

[HGSilveri]

Thanks for looking into this @awennersteen . So far, these changes have not been released yet so I can't tell whether they will help without getting into the weeds of installing qutip from source. As this is not urgent, I'll wait for the release before trying again.