Allow profile indexes to be set as iteration variables

[chris-ashe]

Description of issue / requirement to address

While working on #3256 I noticed that the plasma profile variables alphan ,alphat and tbeta which control the main shape of the profiles (concave/convex etc) were not possible iteration variables.
This removes a lot of possible converged solutions when the solver is trying to converge and only really gives PROCESS the ability to vary the volume averaged temperature or density along with the pedestal and separatrix heights.

This will also allow PROCESS to better optimise on required values for certain bootstrap current and heating & current drive models.

Proposed solution

Implement alphan ,alphat and tbeta as iterations variables and investigate how PROCESS varies these parameters under different regimes and figures of merit