Add cable void variation in stellarator

[jjwalkowiak]

Description

I added an iteration variable to change the fraction of helium (also called void fraction) in the cable/winding pack. The reason behind this is that many stellarator builds are limited by the max B field on coils. To lower max B, PROCESS try to increase the coil cross-section. So far it was possible by increased copper fraction, which led to extremely long quench discharge times. To decouple quench time from max B on coil, I introduced an option to add more void to the cables.

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• 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.