Planning Project Layers Inclusion Testing

[cgearhart]

The layers implementation uses dictionaries for parents/children, so inclusion testing on those attributes maps back to the keys of the layer instead of the values. E.g., foo in literal_layer.children iterates over the literals in the current layer, not the children.

It may be better to make the children and parents private, and expose a read-only getter that requires a key to make the API more clear.