Implement pre-CTE filtering

[JackAtOmenApps]

Add the ability to filter down prior to running CTE.

For instance, in a graph of a municipal district, it may be more efficient to limit the search to a particular region, category of nodes/edges, or other characteristics.

This would involve modifying the raw Postgresql statement to include the additional filtering.

[JackAtOmenApps]

Options to pre-filter on the following:

• A queryset of node instances to ignore
• A queryset of node instances to require
• A queryset of edge instances to ignore
• A queryset of edge instances to require
• A ForeignKey on the node model
• A ForeignKey on the edge model

Maybe in the future:

• A ManyToManyField on the node model
• A ManyToManyField on the edge model