Measure output voltage before output is switched on

[prasimix]

This task is inspired with TSP6 - Rigol DP1116A and DP1308A Programmable Power Supply Review Part 4/6) presentation.

A new option under channel Protection settings has to be added: Prohibit output enable if external voltage detected. Default is ON. It is applicable only to DCP405 power module!

If this option is turned on, the ADC must continuously measure the voltages on the output terminals even when OE (Output Enable) is turned off. If no load is connected through which the output capacitor would be discharged, the residual voltage will be displayed (below OFF text), flashing red while this voltage is greater than 500 mV. Display of the measured voltage will stop when it drops below 100 mV.
When it is active, the output cannot be turned on as long as the measured voltage is greater than 500 mV.

For remote programming we have to add a new command OUTPut:PROTection:MEASure ON|OFF that if activate will generate the following errors when measured voltage is above 500 mV:

510,"External voltage on CH1 detected"
511,"External voltage on CH2 detected"
...
514,"External voltage on CH5 detected" 

[prasimix]

Fixed