`settings.mk` in GNUmakefile.

Bug fix in procedure `dispatch_snapshot`: an isolated eddy is a valid eddy.

In script `plot_snapshot.py`, in function snaphot, replace argument ax
with argument `new_figure` so the user does not have to call figure
and `axes(projection = projection)`. Add argument light to function
snapshot so we can call snapshot with light set to False for a second
snapshot. Add a test to only read `h.nc` when we need it.

In script `read_overlap.py`, print more information: number of nodes,
number of edges, number of nodes with at least one successor, number
of nodes with at least one predecessor, splitting events, merging
events.

In script `stat.py`, use the convention that shapefiles are grouped in
a directory instead of identifying a set of shapefiles by the end of
their basename.

In main program `test_successive_overlap`, we know that `delta_in` for
snapshot 1 and `delta_out` for snapshot 2 are `huge(0)` so do not test
this. Print eddy identifiers in a single line. Add the printing of
identifiers of isolated eddies.