Remove gasmix_is_null

The semantic of fo2==fhe==0 to mean "same gasmix as before" apparently
is not used anywhere and gets in the way of the semantic "this is air".

If there is really need for mix meaning "same as before", please use another
value, e.g. one with a specific negative percentage.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
When adding a waypoint, use the gasmix of the _next_ waypoint.

This corrects the logic of adding a waypoint actually meaning that
a previous segment is subdivided into two. Both new segments should
by default use the gas of the old one. Since the gas is stored
in the waypoint (divedatapoint) at the end, we need to use the _next_
gas.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: reset deco gas used member

Otherwise every recalculation will continue to incorrectly increase the
deco gas used value.

Fixes #598

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>