Update slider/plot dimensions to match cube file dimensions

[BraydenThompson]

References #182

Parameterize slider max values, as well as any other applicable values from 120 (manual width/height/depth of rhopol.cube) to an arbitrary width/height/depth of whatever the current cube file is. (Pull from lengths of cube.data3D maybe?)

[BraydenThompson]

Set slider max to cube file dimensions, although the ipyvolume aspect ratio is still squished for non-cube shaped volumes, and will need much more additions out of the scope of this minor task. Created PR.

[t:65]