- some naming changes on PropComparator, Comparator:

- some naming changes on PropComparator, Comparator:

  1. all Comparators now have "parent" which is always the parent mapper
  or AliasedClass instance

  2. only RelationshipProperty.Comparator has "mapper" now, which
  is the target mapper

  3. The names "parententity" and "parentmapper" are underscored

also improved the message with the "neither comparator nor instruentedattribute...."
to include the classname + attribute name