MAHOUT-1583 cbind() operator for Scala DRMs

This is now ready for review
Not sure why you are saying this. If I am not misinterpreting what you are saying, neither theoretical nor practical estimate supports this. Added an assignment benchmark with and without openhash intermediary. here are the results of running it: 

    Testing started at 12:26 PM ...
    Average assignment seqSparse2seqSparse time: 29.673 ms
    Average assignment seqSparse2seqSparse via Random Access Sparse time: 406.510 ms

This of course assumes that we, for the most part, are having SeqSparse vectors, not RandomAccessSparse ones as payload (which we always are, unless somebody explicitly messes it up with a `mapBlock`.)