Pivot notifications

observe_pivot_fields/3: Foldr to change or add pivot fields for the main pivot table. The rsc contains all rsc properties for this resource, including pivot properties. Fold with a…
observe_pivot_rsc_data/3: Fold over the resource props map to extend/remove data to be pivoted
observe_pivot_update/3: Pivot just before a m_rsc_update update. Used to pivot fields before the pivot itself.
observe_custom_pivot/2: Add custom pivot fields to a resource’s search index (map) Result is a single tuple or list of tuples `{pivotname, props} , where “pivotname” is the pivot…