Rename q to better represent its meaning

[chris-ashe]

This is a follow on from #321 which did a quick variable rename to make the definition of q clearer in more places.

The definition of q does not initially give the impression that it is the safety factor near the edge of the plasma but is instead a function of plasma radius.
Ideas about renaming q to a different variable qedge etc are possible though will make old input files temporarily broken until they are all updated. This definition also changes meaning depending on what model are being ran; stellarator/plasma current scalings etc.

[ym1906]

So there shouldn't be one "q" variable, and have unique ones for each definition?

[ym1906]

Can we close this discussion now @chris-ashe ?

[chris-ashe]

Closed with #3108