Added an XmlUtils object which consolidates redundant code for finding the index of a given XML element.
Moved all the invalid-field checks (and error throws) into Entity. Only one weirdo `_super.set` construct remains.
Adding an XmlUtils.createElement(...) method so the plcOpen namespace isn't copy-pasted everywhere in every file.
Moving common behaviors out of the individual DataMappers and into a generic Data Mapper constructor.