Fix the gas change events for the Heinrichs Weikamp Frog.
Fix the gas change events for the Heinrichs Weikamp Frog.

The Frog stores the index of the initial gas mix at the same location
as the OSTC, but the gas mix percentages are at a different offset, and
the number of gas mixes is different too. Parsing all the gas mixes in
advance makes the code easier to read and more future proof.

glance-/libdivecomputer,venkateshshukla/libdivecomputer,henrik242/libdivecomputer,henrik242/libdivecomputer,josh-wambua/libdivecomputer,josh-wambua/libdivecomputer,glance-/libdivecomputer,venkateshshukla/libdivecomputer,Poltsi/libdivecomputer-vms