Parameter set to improvements

[WilliamHPNielsen]

This PR fixes a bug in parameter.set_to and reduces the amount of instrument communication (calls to parameter.set) that happens in the context manager.

The bugfix: if a parameter has been initialized, but not yet gotten, it will have a _latest["value"] of None. When exiting the context manager, the value that the parameter is restored to should not be None, but instead the result of parameter.get.

The improvement: if parameter has value X, then with parameter.set_to(X) is now a NOOP.

The improvement allows us to write parameter getters that hinge on other parameters without introducing unnecessary communication. An example (simplified) for a DMM that can be in voltage or current mode:

def _voltage_getter(self):
    with self.mode.set_to("voltage"):
        value = self.ask("READ?")
    return value

def _current_getter(self):
    with self.mode.set_to("current"):
        value = self.ask("READ?")
    return value

Imagine getting the voltage or current in a tight loop. In that case, we do not want the communication to be

MODE:VOLTAGE
READ?
MODE:VOLTAGE

for each get call.

@QCoDeS/core

[codecov]

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[jenshnielsen]

Following the merge of #1738 please don't use bare ignores. use --show-error-codes to get the error code needed and do ignore[errorcode]

[WilliamHPNielsen]

Gotcha!