scpi-dmm: add support to get/set range on Agilent protocol using meters

scpi-dmm: add support to get/set range on Agilent protocol using meters

Add support to get and set the auto/manual range on Agilent protocol
speaking devices (34405A, 34465A). The range values are mere numbers
without a unit or MQ associated with them. Support to list available
ranges is prepared but not used (not needed on these meters). Common
logic could open code the lists of ranges from the model description
if desired in a future implementation.