[Subsurface-divelog/subsurface] Fixed get_gas_at_time for equal times (#1835)

[Willem Ferguson]

But while we are at it: Somebody should have a look at the OTU
calculation. For a long time, I have been under the impression that it
gives very different results than other programs computing it...

I can confirm this. I hate to use Multideco as a reference, but if it
might be helpful I can prepare a few plans in Multideco, showing OTU &
CNS, and then do exactly the same for Subsurface. This does not mean
Mutideco does it correctly, but at least one could compare the 2 pieces
of software.

I have not used other logging software for years (Subsurface satisfies
all my dive modes and equipment), but one could do similar comparisons
between Subsurface and, let's say, Divinglog 5. What do you think?

However, I am diving till Sunday, so I could not get to that before then.

Kind regards,

willem