Just updating legacy plotting routines for solved equilibria from FreeGS.

Main changes:

• diverted equilibria separatrix will always be plotted in solid red (small bug meant that some where being plotted with a dashed line when the value of psi_boundary was negative).
• limited equilibria will now be plotted with a LCFS in solid red (to be consistent with the above), but also shown is the separatrix from the primary X-point in dashed black.
• primary X-point is marked with a bold red cross (alongside existing X-points as standard red crosses).
• some legend labels have been updated.
• enabling calculation of Br and Bz derivatives (for use in generating snowflake divertor geometries).

More docstrings also added.