Add support for comparing images

[bnmajor]

As mentioned in #65 it would be useful to have support for comparing images as well

[bnmajor]

This will require a few steps:

• Bump itk-vtk-viewer to latest version
• Add logic for user to specify images for comparison (fixed_image and moving_image)
• Update itk-viewer-material-ui with missing features (select image scale, checkerboard pattern, image mix, window/level, bounding box)

[bnmajor]

@thewtex It looks like from InsightSoftwareConsortium/itk-viewer-material-ui#39 that the intention is to move from the material-ui React library to material-web. In order to get the plugin caught up with itkwidgets (or at the very least just add comparison support) should the missing components just be added with material-web? Or should the plugin changes wait until itk-viewer-material-ui can be updated? Or should we (temporarily) use the itk-viewer-bootstrap-ui?

[thewtex]

@bnmajor last time we checked material-web was not mature enough -- yes it is probably better to use material-ui or itk-viewer-bootstrap-ui, whichever you think is easier.