Fix potential crash in the per tank sac calculation

In an inconsistant XML file (like our own dives/test20.xml) it is possible
that a gas change event refers to a non-existing gas. In that case
get_gasidx returns -1 - which we shouldn't use as index into the arrays.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
TODO notes
Add po2 to CSV-import.

Left to do is CNS, OTU, and make everything except time/depth optional.
Add a travis.yml to test a build.
Revert "Get rid of the planner"

This reverts commit c4a47a7d60b53c9b2e2ffb874afaf733d43123aa.
Preferences: Add some limits to the GF's

Sets the limit for GF's in the preferences panel to 1<=x<=150 and color
it read if gf > 100

Signed-off-by: Anton Lundin <glance@acc.umu.se>
WIP: set backround red on high gf
Playaround with checkboxes in CSV import
Quiet down warning about duplicate name

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Quiet down warning about return in non-void

Signed-off-by: Anton Lundin <glance@acc.umu.se>
WIP