Implement "attributes" aspect

[tclune]

As with the other aspects (Geom,VerticalGrid,Units,...) we need as aspect for "attributes". Here there is no action to provide, but the aspect provides he rules by which "shared" attributes match. (Basically, the dst shared attributes must be a subset of the dest attributes.)

There are a couple of aux things that could be split into separate tickets.

1. I think the existing check on attributes preceded the work that implemented a proper namespace on shared attributes. That should be re-examined and updated.
2. We need to add logic to couplers to propagate shared attributes during the initialization step (We don't allow attributes to be time dependent - at least for now.