Tcx: fix speed in summary information

When exporting files to Tcx, it's writing an ActivityExtension to the end
of the file with calculated AvgSpeed and MaxSpeed. Unfortunatly it's using
the km/h values - while Tcx is using m/sec. Schema is a bit unclear about
this, as the ActivityExtension uses "double" as type for both values, but
original Tcx v2 Schema only uses m/sec, GTC exports as m/sec and some
other software is expecting m/sec, aswell.
Merge pull request #679 from rclasen/fix-tcx-speed

#678: Tcx: fix speed in summary information