add methods to remove groups or units

[rp-]

pydcs mission interface only has methods to add flights/ships/vehicles.. groups, but
to remove groups or units you have to del them on your own from the lists.
But as more automatism was added (callsign id, tail numbers) those set/maps need to be cleaned correctly on deletion.

In the end deletion will not be the norm, as most missions are usually just created from scratch, but
to correctly edit missions this might be useful.