Improve quantity Renderer

[sbliven]

Many fields in the OSC-EM schemas are QuantityValues ({unit:string, value:number}). These are quite verbose with the default JSON Form renderer. We should provide a custom renderer that has the unit and value on one line.

Ideally we would also set a default unit for each field. I looked into this, but it seems that the ifabsent keyword isn't yet supported by the LinkML-jsonschema-generator (see linkml/linkml#2556).