Warn when need for categorical plotting ambiguous

Issue #938 and #940 suggest categorical handling be enable when `color_key specified`
Issue #939 suggests categorical handling should be enabled for booleans e.g. `color=dim('indicator')` where this column is boolean.

However, you can also specify `cmap` with two colors, e.g. `cmap=['#FF0000', '#00FF00']` where `cmap` clearly indicates that a colorbar with smoothly varying values. What happens if `cmap` is specified but categorical handling is indicated by boolean/integer values or the presence of `color_key` as well?

I propose that a warning is issued as the user might have wanted to use `color_key` instead of `cmap` (for instance). That said, I'm not sure when discrete colorbars are appropriate versus a legend...