Improved depth info for dives without samples

Improved depth info for dives without samples

This calculates a mean depth for the dive with a fixed ascent/descent
rate and an assumption that all of the bottom time is at the maximum
depth.  It's not much, but it allows some derived values such as SAC to
make more sense.

The depth profile for such dives is now also generated with the same
assumptions instead of putting the samples at fixed percentages of the
dive duration.

Signed-off-by: Mikko Rasa <tdb@tdb.fi>