Filter which data can be loaded in Views from the main store

A number of algorithms running in EventViews are accidentally loading data objects that exist in the main event store, using the view fallthrough mechanism.

Fallthrough itself is necessary - objects like Identifiable Container Caches must exist in the main store but be accessible in views without creating copies. However, for most objects this is not the case.

So, I have added a filter to the fallthrough mechanism to ensure that only these intended objects are loaded, as suggested by @tbold

Note that this will not prevent an algorithm from trying to load these objects, it will simply create an error rather than an incorrect result. But, at least it'll help us track down the problem.