Associative primer

This PR adds a primer for associative domains and arrays. It is intended to demonstrate features exclusive to associative domains and arrays. 
I'm not keeping this straight and I'm on the development team, so I worry about the reader:  Are all the other set operators available on arrays too?  Are there special rules regarding intersecting index sets for them as well?  I don't feel the need to illustrate every operator on arrays in code, but I think it'd be nice to add a comment after this example summarizing, like "The following operators are also supported on arrays" or "The same rules apply to these other array operators" (if they do), etc.