More consistency improvements

Treat SAC and OTU consistently:
- SAC is now a member of struct dive
- it's calculated / populated at the same time with a helper function with
  consistent API

Create get_volume_units function that returns volumes (e.g. used in SAC
rates) based on preferred units - make sure we have these conversions just
once in the code.

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