lib.xcsoar.analysis: use xcsoar python api for analysis
lib.xcsoar.flightpath: use xcsoar python api for flightpath

Setting threshold to 0 does a polyline reduction based on 'pure'
Douglas-Peucker with no time-biased weighting and returning exactly
max_points points.
views.flight: use xcsoar python api in _get_flight_path

The api has a reversed output of the level of detail, so the client
doesn't have to reverse it itself.