Show Query Set Completeness

[tkw1536]

suggested by @katjabercic:

MDH knows the semantics of any dataset in the system.
One particular information we could keep track of on top of that is if the dataset is mathematically exhaustive, i.e. if all mathematical objects of a particular class are contained in it.
We could then extend this to queries, and let users know if the results for a particular query are also complete.