CellWorx: do not assume that the first well/field file exists

Instead, look for the first file that does exist, and use it to
determine the plane dimensions.
Fix compile error
Fix series index check

The size of 'core' at that point in setId is 0, so use 'seriesCount'
instead.
Merge pull request #686 from melissalinkert/rebased/develop/cellworx-missing-tiff

CellWorx: do not assume that the first well/field file exists (rebased onto develop)