hp-3478a: fix values for digits properties Unify the hp-3478a driver with the other drivers regarding the use of the digits properties in the analog payload. This commit adjusts the previously unclear, but now clarified, use of these properties: Number of significant digits after the decimal point, if positive. When negative, exponent with reversed polarity that is necessary to express the value with all digits without a decimal point.
hp-3478a: Fix switching from 4-wire to 2-wire measurement. The measured quantity flags for the measurement mode can only contain one flag at maximum. Therefore the quantity flags handling can be simplified, which also fixes a bug when switching from 4-wire resistance measurement to 2-wire mode.
hp-3478a: Only match when conn= is given to avoid false probe positives. The HP 3478A device would not reliably identify by means of SCPI queries. The previous scan() implementation would have matched any connected SCPI device, then upset these devices by emitting non-SCPI requests. Tighten the scope of the hp-3478a probe, only scan for devices when a conn= spec was provided. This avoids false positives and malfunction of other devices, and still lets users address the problematic device. This is similar to serial-dmm and unspecific cables and has proven to work there.