slab geometry option

[johnomotani]

It would be useful to have a geometry='slab' option, as well as the current 'toroidal' and 's-alpha'

Prompted by the discussion here #108 (comment).

'toroidal' should actually work for a slab, but it adds stuff that doesn't necessarily make sense. A simple slab probably just wants coordinates x = dx.isel(y=0).cumsum() - dx.isel(y=0)/2., y = dy.isel(x=0).cumsum() - dy.isel(x=0)/2., z = np.arange(0.5, nz)*dz. It can call _create_toroidal_regions() to create regions in case those are useful (although they should be fairly trivial).

To be implemented after #107 and #108 are merged.