Add: Option to automatically calculate max bottom-time based on available cylinders

[Rolf-Smit]

This would require a few things:

• Algorithm: A way to search for this maximum duration that is relatively fast (this can only be done in a smart but brute-force fashion, as a longer bottom-time can lead to longer decompression times, thus gas usage is not easily predicted).
• With multi-level dives: which section of the dive will have this automatic duration calculation?
• With multi-level dives: can multiple sections have this automatic behavior? Would the total duration be equally divided by these sections?
• How would the UI for a feature like this look?

[pabdakine]

Suggest the user would decide a single segment to adjust. A drop-down on each segment near the time input labeled "time" (enabling the input field), "auto-ndl" (field disabled), "auto-gas" (field disabled), or auto-tts" (field enabled, tying in with issue #87). If any segment is set to auto, all other segments are locked at "time".

I wouldn't consider gas for the auto-ndl choice, but the normal warnings would of course be shown if appropriate. The user would then switch to auto-gas (aka minimum gas or rock bottom), but importantly, they now know the NDL time. This would inform the usefulness of trying to use a larger tank. Similarly for auto-tts, the deco gas might be insufficient, but that warning might prompt them to take a larger deco bottle.

Algorithm is simple with a brute force loop eval (although a bisection search would be better). Note each setting would use a subset of possible stopping conditions.