Rearchitect and jit CS fatigue

[je-cook]

Description

I have jitted and reduced recalculation of the same quantities in cs fatigue and reduced calls to np.zeros in pfcoils.
I've also reduced unneeded variable assignment (assigned and then used once) because it is a slow operation in python.
Also * is faster than /

I thought it was worth it for a routine where 50% of time was spent in my run

Speeds up my convergence by >x10 🤯 (2207 iterations -> 134 iterations)

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[je-cook]

from my side is there any reason to write 1.0e0 as opposed to 1.0? I would suggest the latter is clearer if there isn't a reason

[ajpearcey]

Looks good to me