Simplify handling of unit colors

[davenquinn]

Internally, column units are mapped to an enum of predefined color values, that are mapped back to hex codes for the API.

In the future, it might be nice to simplify the handling of this by simply storing hex codes, as is done on the map side. We can write any standard values into the column ingestion UI. The new column visualizations etc. use lithology colors directly, which seems to be far more flexible for standardized coloring than using predefined colors. As with maps, predefined colors can be reserved for "author preferred" column colorings.

@cambro does this seem reasonable? Do you know of applications that depend on the internal units.color field?

[cambro]

units.color was an add because it is typical of many correlation charts to have a color for each unit. I don't feel strongly where this lives or how it is used. I'm sure things like sift probably use it, like my php scripts did.