Refactor altair widgets to reflect that scale and bin can only be used independently

[edwardchalstrey1]

By turning the legend on again, I've discovered that unfortunately setting the type within an alt.Scale object actually sets up a continuous scale which overwrites the bin when both the alt.Bin and alt.Scale objects are used to create the alt.Color object in WAlt.get_altair_color_obj. What this means is that the charts in the notebooks have been using a continuous colour scale that doesn't match what is shown on the "colours used plot" which displays the unused binned colour scale. We should probably update the widget options to reflect that only either "binning" or "scale" can be used at once.

t

[edwardchalstrey1]

This should probably be fixed before we try to make a continuous version of the colours used plot #63