print.c: used gas & gasmix question

[Lubomir I. Ivanov]

hello,

a question about a couple of *cylinder* properties that i found in
print.c:print_tanks() and that are shown in the table that is printed
in the "6 dives/2 dives per-page" mode:

- used gas:
this is done in the lines of:
gas_usage = start_pressure(cyl) - end_pressure(cyl);
using some helper functions local to print.c.

- gas mix:
this is already done in the Qt divelist, but the old print.c does them
slightly differently with:
print_ean_trimix(...)
-> EAN, Tx, Oxygen, air

should these be exposed somehow to dive.h, so that other interfaces
(like the one that does that Qt printing) can access them, or should i
simply double these as helper functions in the new cpp files related
to printing and not add them to dive.h/c?

thanks
lubomir
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