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



This is the same as gh-680 but rebased onto develop.

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If the first file that we're expecting (the one for the first well/field/channel/timepoint) is missing, then look for the first file that does exist and use that to determine the plane dimensions.  This prevents ```setId``` from throwing a ```FileNotFoundException```.