The way external guards are updated (for Fields only!) sucks.  Using relations for this is the wrong way.  It does not permit optimizing the updates as we can with the internal guards.

Solution:
  - Unify dirty handling of internal/external guards. For this
    to work we need to move guard dirty handling from the
    MultiPatch engine down to the Brick (and similar)    engine.
    We get handling of external guards for free, then.
  - We can still use the relation infrastructure to implement
    BCs - just the trigger needs to be elsewhere.  We can
    use a different mechanism to allow BCs for Array, too.