Add score=degrees|kilometers|miles for AbstractSpatialFieldType

Annoyingly, the units="degrees" attribute is required for fields extending AbstractSpatialFieldType (e.g. RPT, BBox). And it doesn't really have any effect. I propose the following:

• Simply drop the attribute; ignore it if someone sets it to "degrees" (for back-compat).
• When using score="distance", or score=area or area2D (as seen in BBoxField) then use kilometers if geo=true, otherwise degrees.
• Add support for score=degrees|kilometers|miles|degrees