Expand functionality of color.method function utilization for hex-plotters

[dtm2451]

Adds capabilities:

1. Use a custom function as color.method for discrete color.var-data
2. Use a custom function that outputs either numeric or discrete type outputs for discrete or numeric color.var-data.

Fixes bug:

• currently / previously, function-type color.method were always assumed to yield numeric outputs. Thus, using a function which returned discrete outputs would ultimately fail later on with the plotting error: "Discrete values supplied to continuous scale."

ToDo:

• Update documentation
• Unit Tests