Summary Link
A User Value Property contains a separate value for each User. For instance, the Rating Property
allows each User to give their own Rating to a given Thing. That is useful, but you often want to be
able to look at the statistics about those Ratings. That is where the Summary comes in.
The Summary stores the aggregate information about the User Value that points to it, and provides you
with functions such as _average to use with that information.
This is a very advanced Property, and you should only use it if you know what you are doing. The Summary's
Type must be compatible with that of the User Value it is summarizing. In the long run, we will wrap all
of this in an easier-to-use UI.