deco calculations

[Dirk Hohndel]

Ok, here's something new...

I took a stab at implementing our own deco calculations.

Jan, don't get your hopes up - it's OC only at this point. CC is on my
list, but not there, yet.

It's a Buehlman ZHL-16b implementation based on code from Heinrichs
Weikamp (as used on their DR5).

I have some issues with this (as noted in the commit messages). My
biggest concern is that apparently changing the GFlow and GFhigh values
has no impact on the deco curve whatsoever. And that is a huge red flag
to me...

So please, give it a try and find the bugs.

(oh, and in order to see any of this - you need to turn it on in the
Preferences / Tec tab)

Thanks

/D