Handle FillColorRGB and LineColorRGB colors in TREx settings yaml

I wanted to use FillColorRGB so figured I'd make a MR.

_get_sample_tuple() and _initialize_unfolding_and_truth_samples_blocks() now handle colors by:

• checking if FillColorRGB is specified, and if so, using that
  • if LineColorRGB is specified, that is used too, otherwise what was specified for FillColorRGB is used
• if FillColorRGB is not specified, a color is picked using the same method as before