BigMatrix should support push(rows)

[MLnick]

BigMatrix supports pulling a set of row indices, i.e. matrix.pull(rows: Array[Long]). For push, only matrix.push(rows: Array[Long], cols: Array[Int], values: Array[V]) is supported.

For common use cases of pull/push a set of rows, this effectively doubles the set of indices that must be sent (2 entries per value). Though I'm not sure if it makes that much difference on performance (it may add a bit to the comm cost of large pushes), for ease of use it would be useful to be able to push a set of rows in the same manner as pull.