Swap the axes of an image using MultiDim

[Cadair]

from #129 it would be nice if you could pick the axes the main viewer displays. I would think this should be easy enough to do with some checkboxes in multidim. i.e.

[X] Axis 0 - [X] Axis 1 - [ ] Axis 2

gives you the standard image

[ ] Axis 0 - [X] Axis 1 - [X] Axis 2

gives you a y-lambda (for a spectra cube) image, then you can control the x position in the same way you currently control the 3rd axis (lambda) position .

[Cadair]

ping @keflavich