11395 fixpyramids (rebased onto develop)



This is the same as gh-1589 but rebased onto develop.

----

Replaces https://github.com/openmicroscopy/openmicroscopy/pull/1486 by suggesting only the use of `bin/omero admin fixpyramids` rather than also the auto-deletion of 0-byte pyramid files from under `/OMERO/Pixels`.

To test:
 * lower the memory settings on Blitz and PixelData to 200MB
 * import 8kx8k.png
 * try to view the image in insight or web
 * A 0-byte pyramid file should be created under /OMERO/Pixels
 * run: `bin/omero admin fixpyramids --dry-run /OMERO`
 * This should say: "Would remove /OMERO/Pixels/..."
 * run: `bin/omero admin fixpyramids /OMERO` which should delete the file.