valType: any is described wrong - and doesn't have enough info to get it right

[alexcjohnson]

Currently valType: 'any' attributes get labeled with number or categorical coordinate string. I'm not even sure if there's an attribute for which that's quite correct - was it meant for axis.range? If so it's missing date strings. But in general 'any' is just used as a catch-all for a variety of attributes that we can't really validate easily, or can take different types in different circumstances. Some of them can be described as an object, with or without some sort of fixed internal structure. What should we do, add a new key to each of these attributes describing its own peculiarities, then pull that key out here?