Since this seems to be specialized for RDDs of ADAMRecord, we should rename to RichADAMRecordRDD or something of the like. However, the name I've just suggested is suboptimal—is RichADAMRecordRDD a rich RDD of ADAMRecords, or an RDD of RichADAMRecords?

Alternatively, it'd be great to generalize this to any class that has referenceIDs; that'd allow merging of all ADAM* RDDs.