get a cell, return default if that cell does not exist
                        note that column and row START AT 1 same as excel

def GetCellValue (self, column, row = None, default = None):
                """ get a cell, return default if that cell does not exist
                        note that column and row START AT 1 same as excel
                """
                if row == None:
                        (row, column) = ParseCellSpec(column)
                if row in self:
                        return self[row].GetCellValue(column, default)
                return default