RFC: Merging hackrf_sweeper into libhackrf

[subreption-research]

What feature would you like to see and why?

We have recently released hackrf_sweeper (https://github.com/subreption/hackrf_sweeper) as a reimplementation of hackrf_sweep. It includes also a 1:1 functional replica of the hackrf_sweep command, besides other demos that could easily be left out of the merge/PR.

The following choices were part of the effort:

• An unused range for errors was reserved so that it can coexist with hackrf_error definitions as-is.
• The sweep-related APIs were all documented in their own group for Doxygen.
• The naming conventions and code style largely match libhackrf's.
• The library itself should isolate the FFTW dependency to itself.

Our preferred merge approach would be to leave hackrf_sweeper as libhackrf_sweep, limiting the FFTW dependency and leaving the core libhackrf alone. This is how it is done presently and could be easily integrated in the CMake process for libhackrf alone.