cubicweb-medicalexp #3702350 Admission etype [validation pending]

We should add an admission etype to model information about a subject entering/leaving a study.

prioritynormal
typebug
done in0.9.0
load0.200
load left0.000
closed by#13ef82313295 [schema] Add an admission etype, closes #3702350
patch[schema] Add an admission etype, closes #3702350 [applied]