New op= for associative domain set operations

Prior to this patch, the op= functions for associative domains were being handled by the generic domain op= functions. Those generic functions don't quite follow the set rules we've decided on. For example, the "-=" operation would halt if the second domain argument did not contain indices that were in the first domain.

Instead, let's write our own op= functions with the desired functionality.
Adding a header comment, such as, "We remove elements in the RHS domain from those in the LHS domain only if they exist.  If an element in the RHS is not present in the LHS, no error occurs."