Control the asked line is ok

def _control_line(self, line):
        ''' Control the asked line is ok '''
        if line > float(self.LINE_LAST_PIXEL):
            return int(self.LINE_LAST_PIXEL)
        elif line < float(self.LINE_FIRST_PIXEL):
            return int(self.LINE_FIRST_PIXEL)
        else:
            return line