Make image optional when initializing PV slice

When making the standalone PV slicer, I found that it would be handy to be able to initialize a PVSlicer widget with no pre-existing data.
As I was working with the PV slice extractor I realized that indeed we currently don't save the WCS for the slice, so it's lost if we want to be able to write it out to a file. I'll try and work on that as part of this PR.