Implement more of the specification.  Also, add a private helper method to avoid rewriting the namespace_prefix/eventually the Solrization information x1000 times.
Accidentally set the expected namespace of the mods:extension element containing NIEC data to niec - reverting.
Hook NIEC extension into the actual MODS datastream correctly.
Unify getter/setter method creation under a single :nattr (nested attribute (maybe not a great name)) class method - eliminates lots of ugly boilerplate code, tbf replaces it with other ugly boilerplate code.