migrate indexing to quakeml

mostly takes care of "preferredXXX" handling for origin, magnitude and
focalMechanism:
- any "preferred" item is used for indexing, if one is specified in the
  corresponding event-subtag (e.g. preferredOrigin)
- if the "preferred" subtag is missing, any first item is used for
  indexing (e.g. first origin in xml file)