function _fnVisibleToColumnIndex( oSettings, iMatch )
	{
		var aiVis = _fnGetColumns( oSettings, 'bVisible' );
	
		return typeof aiVis[iMatch] === 'number' ?
			aiVis[iMatch] :
			null;
	}
Covert the index of a visible column to the index in the data array (take account
of hidden columns)
 @param {object} oSettings dataTables settings object
 @param {int} iMatch Visible column index to lookup
 @returns {int} i the data index
 @memberof DataTable#oApi