add a button to reverse colormap

[agurvich]

as title says, then we can remove all the _r colormaps.

[ageller]

This is working in the colormap_reverser branch. The last step will be to initialize the viewerParams.colormapReversed variable correctly. Currently it is initialized poorly in initPVals and applyOptions .

After that we should remake the colormap image to remove the _r colormaps (and add in more colormaps to replace them).