New split psi plotting option

[clmould]

Expand diagnostic capabilities for equilibria optimisation by adding new plotting option with psi split by contribution.
Figure should include:

• separate plots of the plasma and coilset contribution to psi, for a given equilibrium, in the XZ-plane.
• the location of the LCFS overlaid on each plot.

This plotting option should be available for a given equilibrium object.
This plotting option should be available to display during an optimisation, updated at each iteration.

(Issue copied from Gitlab)

[je-cook]

Merged into feature branch #3338