Support core-only, SOL-only or PFR-only grids

[johnomotani]

At the moment the supported geometries for tokamak grids are single-null and double-null and must include one or two X-points. It would be nice to have more options:

• grid only in the core - should be fairly strightforward. If psinorm_sol < 1 then the separatrix is outside the requested region. Instead of using the separatrix as the base contour use some closed contour (not sure whether the psi_inner, psi_outer or 0.5*(psi_inner + psi_outer) would be best)
• grid only in the SOL - fairly simple again for a single-null; double-null probably needs another option to specify whether to grid the outer SOL or inner SOL
• grid only in the PFR - again need some option to be able to choose to grid lower or upper PFR instead of core