Tests for 'experimental' features

[johnomotani]

A couple of new features have been added recently that are not tested yet. It would be good to add some tests for these, in pytest or in the integrated tests, whichever is most convenient. In suggested order of priority:

• 1. Nonorthogonal X-point (Small fixes to enable grids with nonorthogonal X-point but orthogonal targets #180)
• 2. 'linear' spacing function ("linear" option for nonorthogonal_spacing_method #190)
• 3. Divertor leg extension options (tokamak: leg_extend options #195)
• 4. Possibly the divertor-leg-only grid (Leg only grid #187) - that PR has not been merged yet, so tests could also be added in the PR.
• 5. Any others?