Cellomics: a few fixes for single-file datasets

This allows the field to contain either 2 or 3 digits, and fixes the
image name setting so that field/well indices are not 0.
Cellomics: fix reading of directories with mixed o/d channels

The channel letter for the initial file is respected now, instead of
allowing any file with a matching name.
One more fix for channel file grouping
Merge pull request #968 from melissalinkert/rebased/dev_5_0/cellomics-fixes

Cellomics: a few fixes for single-file datasets (rebased onto dev_5_0)