uhd: new package

UHD is USRP Hardware Driver (for Software Defined Radio).
Add fftwFloat => fftw composition with floating point
gnuradio: new package

Add GNU Radio, the free & open-source software development toolkit that
provides signal processing blocks to implement software radios.
gnuradio: unbreak build (python cheetah module not found)

Something changed in nixpkgs that made pythonFull unable to find
cheetah. Changing from pythonFull to python fixed it.

Just build tested.