Convert physics.f90 to Python

[timothy-nunn]

Description

Converts the remaining physics.f90 module into Python

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.

[timothy-nunn]

When converting bootstrap_fraction_nevins to Python, I have replaced the quanc8 integrator with the scipy integrator. This is because...

1. We cannot use the Fortran solver from Python (it requires a callable)
2. It is ideal to use industry-standard solvers where possible

This causes the unit test, created from values seen in large-tokamak, to change from 886436144167.0579 to 889258771342.7881. @chris-ashe do you have any intuition as to how significant this change is?

[jonmaddock]

Looks great Tim, huge one! Just a couple of chopped-off descriptions in the mfile output, but otherwise fine.

[jonmaddock]

Can't rebase due to conflicts. Can you sort these, then good to go in.