This PR aims to fix #2576 by removing the check on the layers list, while also trying to resolve the issue that the check was put in there to work around. It also makes a couple of other tweaks to the attributes setup. The basic changes are:

• The reference data dropdown now gets updated on a layers callback
• x/y/z attribute choices are now set specifically from the reference data
• We set up default x/y/z attributes for the standard cube pixel coordinate ordering of 2, 1, 0 (currently we are just using 0, 1, 2). I'm not sure if there is any sort of convention for 4D+ cubes, but users can always change attributes anyways