Make the definition of locally definded data elements be part of the configuration

[pkiraly]

Right now the definition of locally definded data elements for MARC21 are Java classes. Yale suggested to make it configurable so libraries can create a Avram/other JSON/YAML/... configuration file which extends the predefined Java classes. The definition in configuration file is preferenced over the Java classes, so e.g. if it contains a definition of a subfield which is available in a Java class (based on LoC definition) as well, QA catalogue will use the configuration.