replace gpxparser.py with gpx.py, which is more powerful
add experimental and untested code to demread.py
Merge branch 'new-gpx-module'

This completely removes the old gpxparser module and repaces it with the gpx
module. The compared to the gpxparser, the gpx module offers:
- more complete support of GPX XML types
- file writing
- better interoperability with guppy classes
fix up new DEM read functions
added some tests for _dem driver (formerly demread) and made sure it passes them
add USGS/CDED DEM to raster formats