ActivityCollection

[sgrellet]

Perspective from discussion going in #2

@dr-shorthair: In practice I have usually found the need for a generic ActivityCollection, which is an umbrella for a set of Observations and Samplings and Treatments (which are a kind of Actuation). Project and SiteVisit are often sub-classes of ActivityCollection.
Probably too generic and too late for OMS, but a bit of perspective.

@sgrellet : Thanks Simon for this. I also tried to push for MonitoringAcitivity during the revision process but failed. It was more a collection of Observations & Samplings without Treatments. But when reading your description I now feel adding Treatment/Actuation in the package would make a lot of sense as we also do have equivalent such Activities here.