Implement TimeBounds() and Bounds() for Gpx and GpxTrk
Add some new functions without tests

Implement CalcMaxSpeed and add Split, Join, LocationAt and MovingData to
Trkseg
Make test function names indicate scope of testing

Test function names indicate also what is being tested, i.e Point,
Segment or Track
Fix MovingData() in GpxTrkseg
LocationAt in GpxTrkseg returns index

Instead of returning a struct, which can be empty, return the index of
the found GpxPoint or -1 if not found. This way it is possible to check
if something was found or not
Implement some functions in Gpx and GpxTrk

Implement Split, Join, LocationAt, UphillDown, Duration and MovingData
in Gpx and GpxTack
Add more output to gpxinfo tool