Add frequencies as optional CRAB parameter

[flowerthrower]

This PR introduces variable Fourier frequencies for the CRAB algorith.
So far they were fixed. This is still the default behaviour and all additional parameters are optional (no breaking changes).
Recently a user requested to provide initial starting values for all parameters. With this PR it can be done in the new QOC package.

[coveralls]

Pull Request Test Coverage Report for Build 8959043767

Details

• 30 of 32 (93.75%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 69.557%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/qutip_qtrl/pulsegen.py	14	16	87.5%

Totals
Change from base Build 8063466154:	0.6%
Covered Lines:	3313
Relevant Lines:	4763

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💛 - Coveralls

[hodgestar]

@flowerthrower Not sure if you intended this for review already. Could you also update the docstrings, add a test and ideally some sort of example to the documentation itself (i.e. under docs/)?

[flowerthrower]

Sorry @hodgestar, I did not intend to mark this as "ready for review" yet: I am not finished working on it.

[flowerthrower]

At this point of time this addition is mostly targeted to make qtrl accessible for qoc.
I designed the change to not interfere with anything existing in qtrl.
To fully integrate this option into qtrl would add considerable effort.
For the time being I would rather point users to use qoc to access this feature.

[hodgestar]

I'm not sure the current solution makes much sense. The frequencies are only present in PulseGenCrabFourier but the fix_freqs option appears on the base PulseGenGran class. Can we move it to the PulseGenCrabFourier class?

It would be good to add a test for the new feature too.

[flowerthrower]

Thank you @hodgestar for your feedback. I have incorporated your suggestions. Let me know if there is anything else!

[flowerthrower]

Dear @ajgpitch, dear @hodgestar.
Could you please verify the recent additions I made?
I want to merge this PR to update the QOC dependency soon.
Thank you guys!

[ajgpitch]

This all seems fine to me, but as @hodgestar has made suggestions (which seem to have been implemented), then best if he approves too.

[flowerthrower]

Some tests are currently failing due to a qutip@master Cython version error.

[hodgestar]

Two more small suggestions made.

[hodgestar]

Some tests are currently failing due to a qutip@master Cython version error.

I reran the tests but the error is still the same. Noting the error message here:

ERROR: Some build dependencies for git+https://github.com/qutip/qutip.git@master conflict with the backend dependencies: cython==3.0.2 is incompatible with cython>=0.29.20,<3.0.0; python_version<='3.9'.

And indeed there is a Cython version discrepancy between the build and install requirements for Python 3.9 in the current qutip master branch:

Install: https://github.com/qutip/qutip/blob/320996bfda3a2eb361eb36cac4208990266b97fe/setup.cfg#L52
Build: https://github.com/qutip/qutip/blob/320996bfda3a2eb361eb36cac4208990266b97fe/pyproject.toml#L7

Note that the build requirements also seem to be duplicated in setup.cfg. Probably worth a least a comment in the file if the duplication is still needed.

@Ericgig Could you perhaps help @flowerthrower sort this out?

[hodgestar]

I'm approving these changes, but let's get CI running and passing before actually merging it so that we've at least had a successful test run.

[hodgestar]

CI tests are now passing. Thansk, @Ericgig.