gr-radar: ported to GNU Radio 3.8

Hi all, 

I've been recently working on porting gr-radar to work to GNU Radio 3.8. These changes can be found in my fork here: https://github.com/primercuervo/gr-radar

If you'd like to play around with it, please checkout the "maint-3.8" branch and install in on top of a working GNU Radio 3.8.

I've limited my scope for testing this to the UnitTest available and the simulation flowgraphs (which are in itself extensive and provide a lot of coverage). However, I have no hardware to test the over-the-air part of this OOT, so this is still lacking. All the flowgraphs, regardless of their HW usage, have been ported to yml as well. 

In the meantime, I'll prepare a PR to the main repo. Any type of feedback is welcome! 

Cheers,

-Nico