Patching small issue with dr_sep for some equilibria

[kpentland]

Just patching a small issue that returned an incorrect value for dr_sep for some "weird" shaped equilibria. Should work fine for most but this should now work for all.

Main changes:

• instead of filtering the x-points by those that have psi values closest to psi_boundary, we instead keep only those inside the tokamak wall, then choose the two closest to the magnetic x-point.
• the values of psi at these two x-points are then used to find dr_sep.