strassen-gpx.t: new abstraction xpath_checks()
Strassen::GPX: trip extensions

Produce Garmin's trip extensions (currently only ShapingPoint
and ViaPoint elements) if -withtripext => 1 is specified.

Both XML::LibXML and Twig implementation can handle this.

Added test cases to t/strassen-gpx.t

BBBikeGPS.pm is prepared to use this, but this is not yet activated.
data update