refactor: rename coordinator to harness

the coordinator/crossbar combination of names was a bit
misleading as crossbar was the coordinator and in fact the
coordinator package was a harness that combined the local
coordinator and exporter as a context manager to be used in
other code. This refactor makes that distinction more obvious
and also paves the way for the renaming of crossbar to
coordinator as crossbar is no longer supported in `Labgrid`

BREAKING CHANGE: `coordinator` renamed to `harness`