This isn't actually the data that callers usually want. Most of the
callers just want the bounds. The heading and length are trivially
computed from that. Add a class to contain the result so it's easier to
refactor.
It replaces the FRONTLINE_LENGTH constant, which was used previously. The default setting for the frontline length has been set to match the FRONTLINE_LENGTH value (80 km), but I feel the default should be a bit shorter. Discussion on the topic would be welcome.
This is briefly moving us over to my fork of pydcs while we wait for
https://github.com/pydcs/dcs/pull/206 to be merged. The adaptation is
invasive enough that I don't want it lingering for long.
