feat: allow for coloring by both flux and redshift

[bgmeulem]

Currently, the black hole images are by default colorized by their flux value (high flux = white, low flux = black).
There is support to plot by redshift as well:

bh.plot_isoradials(color_by = "redshift")

This feature is for now only reserved for a dedicated call to color_isoradials, and the default method plot selects flux, with no option for the user to override this.

It would be nice to allow colorizing by both flux AND redshift. This would need a dedicated color module to translate properties like flux, redshift AND density (see #56 ) to a single 4-d color.
My initial thought would be to :

1. translate redshift to a color between blue-white-red
2. translate flux to an opacity value
3. flatten the color with black as a background color to an opaque value again
4. use density as a final opacity color