Proper support for some non-numeric 'third data dimension' values in ScalingRelation

[DavidT3]

Currently it only properly supports numeric data, as the third data dimension property attempts to put the data in an astropy quantity, which will only work for numeric data.

Would probably be pretty great to let the points be coloured by string labels, or something like that. That way classes of object can be highlighted, and it would be more generalised than what is currently possible (you can pass True or False values, as they can be numeric).

The colorbar setup would have to get more sophisticated though, as would colorbar tick labels. Labelled points, rather than values which are on some continuous scale, would have to have a segmented colorbar set up instead