Add rigol dm3000 scope

Add RigolDM30xx-type digital multimeters, as found in OPMD laboratory at Oxford (see programmers manual).

RigolDM30XX.cpp and RigolDM30XX.h are heavily based off corresponding files for the Keithley2000-series, and appropriate stubs have been added to libMeter/python.cpp. CMakeLists.txt has been adjusted to build the new device.

Operation has been tested in Oxford OPMD for the basic operations (measure I, measure V, measure R, measure R 4-wire, identify, reset).

Open questions:

• The Keithley2000 implementation performs a reset (*RST) before every measure operation. On the Rigol, this is very slow (~1-2s); is this considered strictly necessary from the LabRemote side (I assume expecting the MM to be in a potentially unstable state at the start of every operation), or would it make more sense to avoid in this case?
  ---> the reset commands have now been removed on advice from Elisabetta