scpi-dmm: use different Agilent SCPI command for range and measurment

scpi-dmm: use different Agilent SCPI command for range and measurment

Don't use the "[SENS:]<mode>:RANGE..." request on Agilent protocol
speaking devices. Use "CONF:<mode> [<range>]" instead. This was tested
with all devices which reference this model.

In theory there is the question whether previously configured ranges
should be re-used when (exclusively) the MQ is switched. It's assumed
that this is not useful, instead users or their applications should
select the function first and optionally pick the range then. Re-use of
e.g. 10k resistance range when switching to capacitance might be seen as
confusing (and results in errors in the device).

Use the combination of INIT and FETCH? to get measurements. The READ?
request is more complex and might "undo" what INIT already initiated.
This improves perception of the 34405A meter's operation.