lib.xcsoar.analysis: fix flight detection algorithm
lib.xcsoar.analysis: fix flight detection algorithm

Add some fallbacks for failed flight / scoring window detection

RBE-Avionik/skylines,snip/skylines,Harry-R/skylines,Harry-R/skylines,kerel-fs/skylines,Turbo87/skylines,TobiasLohner/SkyLines,skylines-project/skylines,Harry-R/skylines,skylines-project/skylines,snip/skylines,TobiasLohner/SkyLines,kerel-fs/skylines,skylines-project/skylines,TobiasLohner/SkyLines,Turbo87/skylines,RBE-Avionik/skylines,shadowoneau/skylines,skylines-project/skylines,shadowoneau/skylines,Harry-R/skylines,shadowoneau/skylines,Turbo87/skylines,snip/skylines,kerel-fs/skylines,RBE-Avionik/skylines,shadowoneau/skylines,RBE-Avionik/skylines,Turbo87/skylines