MacDive import improvement

MacDive reports non-existent values as zero and in Imperial units it
would be 32F. This patch will try to make an educated guess whether we
have the proper temperature and pressure readings and ignore the zero
fields if it seems that there are no real samples from the dive
computer.

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