Fix type inference on padarray
Change color slices to colorspace "Unknown"
Allow permutedims to work with tuple inputs
Overlay: default clim using limits, delete limits & colorspace properties

Also try harder to return an image from OverlayImage
io: for multi-image files, switch to interpreting as a timedim rather than spacedim

The motivation is that multi-image files are more commonly temporal sequences
(Think of all the GIFs posted online...)
If this breaks your code (sorry!): manually adjust the properties after calling imread.
Fix unit parsing in NRRD