Support subset selection for annuli and split mesh
Currently, there is no way to distinguish between the inner and outer cylinders.
This will require an additional label in the FixedObstructionGroup to store the name (e.g., :inner or :outer for annulus or index of split mesh).
This could be accessed in Subset
by setting geometry_index
to a tuple with the index of the annulus/total mesh as well as the label of the selection within the annulus/total mesh.
From the command line it would probably makes most sense to write this as --subset inside 1.inner
to get the spins inside the inner cylinders of the first obstruction group (which should be a set of annuli).