docs: public void copyStructure( DMatrixSparseCSC orig ) {
          reshape(orig.numRows, orig.numCols, orig.nz_length);
          this.nz_length = orig.nz_length;
          System.arraycopy(orig.col_idx,0,col_idx,0,orig.numCols+1);
          System.arraycopy(orig.nz_rows,0,nz_rows,0,orig.nz_length);
      }