Reorganize code for creating scale objects
Remove unused scales-related code
Switch from set_dscale to scale_numeric, scale_ordinal, etc.
Rename remaining instances of scale class to vega_scale
Rename default_scale to default_vega_scale
Reorganize scale functions
Fix function name
Add documentation for scales
Re-document
Update demos to use new scales API
Update remaining uses of set_dscale to new API
Merge branch 'scale-refactor'