Currently, the iOS version of Subsurface does not allow users to input
an average depth when manually adding a dive. This prevents proper SAC
(Surface Air Consumption) calculation, which depends on the average depth.

This change adds:
- Average depth input field in the dive edit UI
- Model support to expose average depth data
- Display of average depth in the dive view
- Backend validation and storage of average depth
- Preservation of user-entered average depth when generating dive profiles

The implementation follows the same pattern as the max depth field and
allows users to clear the average depth by leaving the field empty.

Signed-off-by: Michael Tanzer <mt.michael97@gmail.com>