Fixes bug in rx voltage measurement

ohmpi/hardware_components/ohmpi_card_3_15.py

@@ -254,6 +254,6 @@ class Rx(RxAbstract):
         """ Gets the voltage VMN in Volts
         """
         self.exec_logger.event(f'{self.board_name}\trx_voltage\tbegin\t{datetime.datetime.utcnow()}')
-        u = -AnalogIn(self._ads_voltage, ads.P0, ads.P1).voltage * self._coef_p2 * 1000.
+        u = AnalogIn(self._ads_voltage, ads.P0, ads.P1).voltage * self._coef_p2 * 1000.  # TODO:  check if it should be negative
         self.exec_logger.event(f'{self.board_name}\trx_voltage\tend\t{datetime.datetime.utcnow()}')
         return u