Write our own versions of op= for associative domain set operations instead of relying on the default ones
Add a test for domains with no overlapping indices
Adding comments stating how set operations on domains handle the removal of indices not present in the LHS domain.
Merge pull request #428 from benharsh/assoc-opeq-fix

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.

The opEquals.good file is changed as serialization warnings were no longer occurring.