Review Transformer Model used in pfcoil module

[ajpearcey]

Original Issue #101

Consider whether transformer flux swing calculation can be improved by using the mutual inductance between the Central Solenoid
and the plasma - which is already calculated.
The description in the Physics paper is as follows, but no justification is given:

Papercut Review

We should review the models in the pfcoil module are they are quite opaque and current documentation is not complete. Then we can perform a literature review of other possible models

Also we should highlight that PROCESS performs a calculation for both the volt-secs produced by the coil set and compute the volt-sec required for the plasma physics solution. We have a few open discussions on these issue (eg #2884), and it would make sense of consider these issues together.

[timothy-nunn]

Worth checking the bit of the code that uses a single value decomposition. When moving to the scipy implementation we got different results. I also showed that the in-house PROCESS algorithm was wrong #3418 (comment)