uhd: Updated docs, in particular for the command interface
uhd: Added command interface to uhd source, modified command syntax
uhd: Refactored common stuff from usrp sink and source
uhd: Added GRC bindings for command interface
uhd: Added more type checking and flexibility to commands and tags
uhd: Added example for re-tuning through messages
uhd: Added checks for lock sensors.

When operating with an external ref, in MIMO mode or with a GPSDO,
the constructor now waits until the lock sensors report succesful
locking.
If this fails, a warning is displayed using the logger interface,
to keep backward compatibility.
zeromq: updated zeromq examples to use qtgui.
fcd: updating FCD examples to use qtgui.
Merge remote-tracking branch 'mbant/uhd/msg_format' into qt_the_things
fec: updated fecapi blocks in grc to use new var_value concept.