Modify v.out.gpsbabel and v.in.gpsbabel to manage track and waypoint elevations

[needelsd]

GPS tracks/waypoints generally contain elevation data, as do GRASS 3D vectors. Currently elevation data in either format are ignored when translated to the other. Include a flag to force v.out.gpsbabel to export elevation data from 3D point vectors to waypoints or from 3D line vertices to tracks. Ditto for v.in.gpsbabel in the other direction. Note that the standard units for elevation in a .gpx file is in meters, so provide a mechanism for specifying the elevation units in the GRASS location and convert to & from meters as required.