Event-model

Added
*Added data_session and data_groups to Run Start document schema.

Changed
* The package requirements have been relaxed to accept jsonschema versions 2 or 3. Both are supported.

* The "projections" schema in the Run Start document has been enhanced to accept "configuration" locations intended to link to fields that are in Event stream Configuration See :ref:`projections`. Additionally, validation of projections was enhanced.
* The method https://github.com/bluesky/event-model/blob/b7a5fd8f54a9ee96c33a2171e8ecb243f0b08391/event_model/__init__.py#L1283 was enhanced to check for repeated runs with colliding uids, raising an ValueError when this occurs.
[event-model-1.16.1.tar.gz](https://github.com/bluesky/event-model/files/5550012/event-model-1.16.1.tar.gz)
[event-model-1.16.1.zip](https://github.com/bluesky/event-model/files/5550013/event-model-1.16.1.zip)

Added
* The schema for Run Start documents now includes an optional “projections” key. See Projections (Experimental).
* Added the method emit and an emit parameter to DocumentRouter to support chaining them.
* The Filler now provides public methods for clearing its caches, clear_handler_cache() and clear_document_caches().
* The method deregister_handler() returns the handler that it has deregistered.
* The filler_state attribute of Filler now includes attributes resource and datum which may be used by coercion functions to work around incorrect shape metadata.

Changed
The function register_coercion() replaces the misspelled register_coersion(), which is retained as an alias for backward-compatibility.