Add TTS or Minimum Gas (bar)

[sjjan]

If you use 2 dive computers, you can also use TTS and Minimum Gas (in bars) as a planning guide. You start your ascend whichever comes first: TTS or the x bar pressure. Would be nice if this can be added.

[Rolf-Smit]

Note: the 'minimum gas' part is similar to: #29

[pabdakine]

For min-gas, the user wouldn't specify a single pressure. Rather, a computation based on the stressed SAC would give the min pressure.