Improve logging

[michaelaye]

Currently, I get a slightly confusing logging message twice for the same CalBlock but for a different set of moving_only samples:

INFO:diviner.calib:Calibrating from 2017-08-01 00:00:01.411000 to 2017-08-01 03:00:00.533000.
WARNING:diviner.calib:No caldata in calib_label at 2017-08-01 00:00:01.411000. Found only 64 moving samples.
WARNING:diviner.calib:No caldata in calib_label at 2017-08-01 00:00:01.411000. Found only 96 moving samples.
WARNING:diviner.calib:No caldata in calib_label at 2017-08-01 00:00:01.411000. Found only 64 moving samples.
INFO:diviner.calib:Found less than 2 SPACE labels in CalBlock at 2017-08-01 02:59:40.180999.
INFO:diviner.calib:Performing radiance correction on 2017-08-01 00:00:01.411000

This should be improved somehow...