Slidebook TIFF: prevent NPEs when metadata is missing
Slidebook TIFF: close open file handle in TiffParser
PNG: fix support for images with fewer than 8 bits per pixel

Fixes #10887.
Metamorph TIFF: populate PhysicalSizeZ

Fixes #10884.
Leica LIF: parse PhysicalSizeZ from one more place

Fixes #10843.
DNG: fix type checking to prevent picking up Photoshop files
SVS: disable file grouping

It's very common to have similarly-named files in the same directory
that have very different dimensions.  To prevent weird problems with grouping,
and because the workflow of dealing with slides means that grouping
isn't really necessary, it is now disabled.
Slidebook TIFF: make type checking more rigorous
CellWorx: do not pick up .pnl/.tiff files if file grouping is turned off

Noticed by Stéphane Dallongeville.
Leica LIF: a few more physical size and position fixes

Noticed by Olivier Burri:

http://www.openmicroscopy.org/community/viewtopic.php?f=13&t=4710#p9964
CellWorX: one more type checking fix

This allows the log files to be picked up if isGroupFiles() returns false.
Leica LIF: prevent indexing issues if an invalid field position is found
Metamorph TIFF: fix compile errors
Merge pull request #609 from melissalinkert/may-bug-fixes-dev_4_4

Assorted bug fixes for May (rebased onto dev_4_4)