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.