Include TableTableRowGroupElement support in all row based methods

[YSavanier]

Hi
Actually the only method supporting the presence of the TableTableRowGroupElement is the getNextRow and getPreviousRow of the OdfTableRow class.

Others classes like OdfTable don't manage to get the real number of rows with getRowCount or iterate among them using getRowByIndex or getCellByPosition, they all fail to considerates TableTableRowElement nodes contained by a TableTableRowGroupElement.

Best regards

[svanteschubert]

Unless someone is providing a patch, I will take this into consideration for 1.0.0.