MAHOUT-1596: implement rbind() operator

Signed-off-by: Anand Avati <avati@redhat.com>
style: (1) we don't use ( in closures, (2) we try to use -> here, i.e recommended style to reduce amount of punctuation: 

    // Adjust RHS keys
    val bAdjusted = b.map { case (key, vec) => (key + n1) -> vec }
   
    // dump a and b together 
    ....

also definitely needs more comments on closures