Try to detect whether DivingLog's samples are m/f

Seems that we can detect the unit of samples from DivingLog import when
comparing the maximum dive depth to the maximum recorded sample depth.
If sample is bigger, it is recorded in imperial. Of course this relies
on the assumption that the max depth of dive is recorded in metric unit,
but that seems to be the case, at least in the sample received.

It really does not make any sense to have both units in use for one dive
without any info on the unit used. But since that is the case, let's try
to cope with it.

Fixes #316

Signed-off-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>