v0.0.3

This release adds some essential features to the API:

• Implemented a high-level annotate() method analogous to foliapy's add() method. Handles span annotation transparently.
• implemented common_ancestors()
• implemented get_layer_key()
• make sure layers reverse-inherit their set from their children
• finished provenance support, an active processor can now be associated with a document
• datetime attributes are now chrono::NaiveDateTime structs and properly parsed, rather than a plain String