Rohde&Schwarz HMC 8043 support

libsigrok now supports the Rohde&Schwarz (previously Hameg) HMC 8043 programmable power-supply.

This is a 3-channel power-supply with USBTMC and LXI connectivity.

The protocol is SCPI-based, so the patch for libsigrok was relatively straight-forward, extending the existing scpi-pps driver.

Other, similar SCPI-based power-supplies could be added with relatively little work as well. Patches are always welcome!

The code was contributed and tested by Marc Schink, thanks a lot!