Gossen Metrawatt Metrahit multimeter support

A few weeks ago libsigrok received support for various Gossen Metrawatt multimeters, a line of DMMs we didn't yet support at all.

The respective gmc-mh-1x-2x driver (for the Gossen Metrawatt Metrahit 1x/2x series multimeters) was contributed by Matthias Heidbrink (thanks a lot!).

It already supports a number of devices, e.g. the

Other devices are already being worked on, and/or can be added to the driver by interested users (patches welcome!).

The driver was tested with the Metrahit RS232 PC interface, support for other interfaces may follow (again, patches and people who can test with these interfaces are highly welcome).