Add: Option to add gas switch time to the dive plan

[Rolf-Smit]

Currently gas switches are assumed to be done almost instantly or while traveling, however when ascending it makes more sense to add a flat section of a minute or so to allow some switch time.

• Configurable between 0 and x minutes?
• Only for gas switches while ascending.

[pasqualtroncone]

Configurable between 0 and x minutes?

This would allow for both scenarios:

• 0 minutes for cases where the gas switch is done on the ascent without stopping.
• 1+ minutes (default) for cases where a stop is required to switch gases.

In the 1+ minute scenario, the gas consumed during the stop should be included as part of the gas calculated for the ascent up to that point. In other words, when planning a gas switch, I assume the switch will take 1 minute per person (2 minutes total for my buddy and me). While my buddy performs the entire switch sequence, I continue breathing from my back gas until it’s my turn, and then I keep breathing from it until the very last moment—when I switch to the stage cylinder regulator.

This gas consumption should also be factored into the emergency gas calculation, which is currently not considered by Subsurface.

Hope this helps

[Rolf-Smit]

Fixed by #90

Notes on gas consumption:

The current implementation assumes you keep breathing from your current gas until the configured switch time ends before making the switch. This is the more conservative approach: you start breathing from your better gas later rather than sooner, so the higher off-gassing gradient kicks in later, and the resulting decompression is slightly longer. If in reality you switch sooner, that is only beneficial, as you will be out of decompression ever so slightly sooner then planned.

The configured gas switch time does not account for your buddy or your team, since the app has no knowledge of how many divers are in the group. It is worth thinking carefully about the time you configure, a larger team will likely need more time than just you and your buddy.

Emergency gas usage is taken into account, since it always calculates from the worst-case point in the dive to the surface. It also always assumes you and your buddy are sharing all gases along the way, so gas switch sections are included normally. In practice this means both divers are assumed to switch to the better gas at the end of the switch duration (not 100% realistic, but well within acceptable margins). Any more precise model would add significant complexity for negligible gain.