dynamically sets colspan in a row starting from the specified column index
| row_id | string| number | the id of a row |
| col_index | number | the index of a column |
| colspan | number | the size of colspan |
Available only in PRO Edition
mygrid.enableColSpan(true);
//merge first 2 cells in the row with the id "row1"
mygrid.setColspan("row1",0,2);