make colormapping respect choice of field in GUI

now, how to distinguish standard "image" fields, new "value"
fields and "support" fields like normals.
recover layer and field to use to choose colormaps

now it reverts to drawing old "image" fields directly and when given
a "value" field maps it through the lookup table and applies lighting.
Demonstrated the use of different color maps for different fields by
hardcoding the 'vBrownianVectorsX' field to use a hardcoded rainbow
color map. In principle we can have as many LUTs as we have value
fields now.
avoid in band signals, per review comments
hook up lut to each field and fix a small bug

Specular lighting was off where normals pointed away from light. Now
it the specular component is 0,0,0 there (self shadowed).

The bigger change is that now each field has its own lookup table.
You can choose rainbow for salinity and spectral for temperature now
for example. The rendering widget edit section gives you a choice of
the available value fields (from the new FieldsModel) and the changes
you make in the rest of it pertain to the LUT associated with it.
fix style checks on code I touched
lighten up a little

changed ambient behavior to bring in value color and changes phong
constants to make it less dark.
make lookup table text fields correspond to field ranges
Merge branch 'colormap-chosen-field'

Conflicts:
	web/src/js/lib/widgets/CompositeToolsWidget.js