data structure cleanup

[Dirk Hohndel]

These patch series are incredibly boring and yet have a high risk of
breaking things - so please test Subsurface in general, not just adding
plans (admittedly, that's where most of the changes are visible).

I have made decent progress towards making us actually use our gasmix data
structures in more places. This is far from done, but I wanted to push out
what I have so that testing can start :-)

/D