Changed the way dive data points for OC cylinders to be added to the
dive plan are created in `createTemporaryPlan()` in
`diveplannermodel.cpp`. This now uses `plan_add_segment()` like all
other places where dive data points are added, in particular the planner
tests.
This also allowed for `create_dp()` to be made static.
Signed-off-by: Michael Keller <github@ike.ch>