Add dictionary of transfer function presets

[swederik]

We will want to add some UI elements to change the transfer function of the rendered viewports. We don't need to add the UI elements as part of this issue, but we should add some functions which makes it easy for this element to be added.

• Please copy the basic VR Presets from Slicer (https://github.com/Slicer/Slicer/blob/67c4ae61b41ea7a2382e44284d89c41c75ba2aff/Modules/Loadable/VolumeRendering/Resources/presets.xml#L3)

[pieper]

Note that the vtk-js VolumeController already includes the transfer functions from Slicer along with a bunch of other ones. It's probably good to use the vtk-js widget directly and either improve it or swap out later if we don't like it.

[DominionSoftware]

As far as I can tell this doesn’t have much in the way of HU medical transfer functions and has a overabundance of false color look up tables.Also the opacity is not clearly usable. Histogram doesn’t show up at least I don’t see very well. Should we (i.e. I) I take the time to crib this code and expand upon it ?