A ground station's access schedule is one search per satellite, each
independent of the others, and a shell gives enough of them to pay for the
threads. The parallel form returns the same summaries in the same order as
the sequential one, including an error at the position the sequential form
would put it, so one bad track does not hide the rest.
Thirty four crates listed apsis-core or apsis-math without referencing it,
and apsis-kepler listed apsis-time and apsis-tle as well. Each was verified
removable by compiling with it taken out. An unused dependency still costs
build time and makes the dependency graph and the layer check report
relations that do not exist. The layer check now reports 104 edges, down
from 141.