lib.xcsoar_.flightpath: use a MultiPoint geometry instead of LineStrings

The MultiPoint geometry can handle geometries with a single point only.
frontend.views.flight: work around broken flights with takeoff_time > landing_time

This should never happen, but earlier versions of AnalyseFlight
has returned such broken output, so we have to deal with it until
there's a better solution.
Merge pull request #259 from TobiasLohner/xcsoartools

Fix some minor bugs of the new xcsoar python lib usage.