Removed tfsai and tfsao from sctfcoil due to obsolete var in calcs

[j-a-foster]

Description

I removed tfsai and tfsao from sctfcoil.py as they were using the variable hr1 which wasn't calculated or an input and just set to 0. A power calculation that used tfsai was then replaced with tfcryoarea based on comments in the code (which have now been removed).

Checklist

I confirm that I have completed the following checks:

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

[j-a-foster]

The changes in the regression tests appear to make sense to me but would like a second opinion.

[mkovari]

Since the cryogenic model is pretty rubbish anyway, I don't think this deserves too much brain power. In any case it's got to be better (?) to replace a variable that was never set by one that has a value.

[mkovari]

Please delete the branch if you are sure I haven't done something stupid.