Add SCPI watchdog commands

[fietser28]

This is a reliability/safety feature when using the BB3 in combination with a remote program via SCPI:

In some cases (e.g. charging batteries) it is important the BB3 disables its outputs if the connection with a remote SCPI script is lost. This is like a watchdog or a keep-a-live message.

I propose to implement a few SCPI commands to enable a watchdog like behaviour: If the SCPI watchdog is enabled and there is no SCPI activity for a defined period of time the outputs will be disabled (just like inhibit or protection).

Existing examples:

• keysight
• electronic load see input:wdog commands.