Planner: only track gas when planning a dive

When adding a dive we don't have SAC data so we should simply have the
user manually enter their gas consumption. That still doesn't work, but
this is one required step to get it to work.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Planner: don't set cylinder start and end pressure when adding dive

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add dive: don't throw away the pressure data entered

Previously we were trying to get rid of the tracked gas pressures that
were added when using the plan infrastructure to create a dive. Now that
this isn't the case anymore we can stop doing that which magically makes
setting start and end pressure while adding a dive work as expected.

Fixes #582
Fixes #553

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