Motivation for this change

Add a new package, dsd, which is a common ham radio library for decoding digital voice:

DSD is able to decode several digital voice formats from discriminator tap audio and synthesize the decoded speech.

Also add the two required libraries that aren't already in Nixpkgs: mbelib, by the same author as dsd, and IT++, a C++ signal processing library (http://itpp.sourceforge.net/4.3.1/)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Picking a few reviewers for things that are SDR/radio-related:
r? @bjornfor @fpletz @alkeryn