cost is Argument, but argument does not have to be cost.

* also remove unused getValueStr

* hidden decorator kwargs in DataProvider.__init__
* also add unit test for this.

* hidden decorator kwargs in DataProvider.__init__
* also add unit test for this.

* hidden decorator kwargs in DataProvider.__init__
* also add unit test for this.

Change float in test_ConvTrans and test_ConvUnify to real.

* The DataProvider should be INCREF every time.

* The original SparseRowMatrix use two fields to store each rows,
  which let code very confusing. Try to extract a RowBuffer class,
  for SparseRowMatrix data storage, and manage auto-growth logic.