Removed the requirement that the relationship for ACL associations be
"isRelatedTo". Now the relationship in the triple can be anything, like
"describes access control rules for". This doesn't affect the ACL
processing because it is based on the ACL document type, not the
relationship. This will probably need to change in the future, because
we'll need to support multiple ACL doctypes as different versions of EML
are created. Jivka -- could you review this change please?