calculator.power_spectrum asks for c_q instead of calculator's rotation key

[rosecers]

Not all calculators will use c_q for rotations and power_spectrum should listen to that

[arthur-lin1027]

Yes, this was a design decision Seonwoo and I made, but I can revert this to take in any key if that makes it more convenient.

[arthur-lin1027]

TODO

• Add test for power_spectrum ensuring that it can accept any quat key.
• Allow power_spectrum to handle matrix rotations too?

[rosecers]

I would suggest that power_spectrum has a rotation_key input as well, and then checks for that key specifically.