GFT: Add new Shadows/Open Space dataset

Related to #664.

According to the GIS spreadsheet, a new shadow/open space variable Schoolyard-to-playground should be added to GFT.

Add new template for dpr_schoolyard_to_playgrounds dataset in data library. Extract data
Add staging & intermediate & product models for dpr_schoolyard_to_playgrounds.
- For intermediate, create buffered model of dpr_schoolyard_to_playgrounds in intermediate/buffers. Add that model to buffers_all
- For product, create 2 buffered and unbuffered models of dpr_schoolyard_to_playgrounds in product/sources
Update variables seed file to include the new variable
Export the dataset

Could you please help me confirm I added dpr_schoolyard_to_playgrounds to all required product models? And nothing else needs to be exported?

NYCPlanning / data-engineering