Merge branch 'trips' of git://git.hohndel.org/subsurface

Merge the initial 'track trips explicitly' code from Dirk Hohndel.

Fix up trivial conflicts in save-xml.c due to the new 'is_attribute'
flag.

* 'trips' of git://git.hohndel.org/subsurface:
  Fix an issue with trips that have dives from multiple input files
  Some simple test dives for the trips code
  First cut of explicit trip tracking