Add concrete scale implementations that support CGFloat as an output type

[heckj]

For the work on Chart (specifically the VisualChannel construct), we have a couple of types that aren't currently covered which should be:

The (default-ish) output type needed to draw into a GraphicsContext is CGFloat - for better or worse, so we probably need explicit implementations that output there instead of (or in addition to) the current Float. That may mean we want to reconsider how those mappings are happening, not sure.

In addition, there's a need for scales that convert values (Int, Float, Double, etc) into colors. I suspect that's probably a variation of LinearScale, but the interpolated output should be a type of color, or a type that can be easily converted into a color. First cut pass, maybe CGColor from CoreGraphics, but a general color representation might not be amiss that can apply over into SwiftUI color, UIKit/UIColor, or AppKit/NSColor as needed.

Finally, we need some discrete scales - none currently in play, to enable bar charts - something that takes a list of categories (represented by a [String]) and can return a spacing location for the center, or be used to calculate the relevant positioning with a width and spacing. So again, output to CGFloat, input a Collection - where first cut of Something is String

The secondary output type for the discrete scale is color - to enable color to be used to provide an indicator of an underlying category choice in things like Dot or Line charts.

[heckj]

(probably want to break this up into separate issues and sections)

[heckj]

Broke out color scales (#6) and discrete scales (#7) from this issue - enabling CGFloat as the output type for existing Linear, Log, and Pow concrete scales