The EVENT_CONTEXT class needs some adjustments to better
correspond to the reality it models. Firstly, 'author' should be
renamed to 'composer', to correspond with the emerging CEN
Secondly, the health_care_facility attribute should be optional,
since a care event could occur at the patient's home, with no
involvement at all by a care delivery enterprise (e.g. creation
of some observations; self-administration of a drug).