Replaced local calculated eps variable with fortran variable as input

[j-a-foster]

Replaced inverse aspect ratio calculated locally in _plascar_bpol with input eps which is the fortran variable physics_variables.eps in subsequest functions which use _plascar_bpol. These functions are bpol and plasc.

Closes #454

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.

[j-a-foster]

Unsure whether need to change physics.py line 751 as it's calculated on an index by index basis. It's also not clear from the docstring/comments whether this eps corresponds to the eps mentioned in #454.