Fix multi-file export in ImageJ

Closes #9829.
Cellomics: only do a directory listing if we are grouping files
TileStitcher: attempt to stitch even if PositionZ varies

Fixes #9378.
Zeiss LSM: make the .mdb file checking a bit more strict

Fixes #9807.
Merge pull request #318 from melissalinkert/pr201-develop

PR 201 on develop