Add CGRAN support to gr-sigmf

[pwicks86]

It would be nice if gr-sigmf showed up on CGRAN. We should add pybombs support so it does.

[mattes-bru]

What is blocking this? I can provide a pybombs recipe, if needed.

[pwicks86]

I ran into some trouble building a working pybombs recipe and hadn't had the time to go back and investigate further. If you have a working recipe, I'm happy to accept it for gr-sigmf so we can close this.

[mattes-bru]

This recipe works for me:

category: common
depends:
- gnuradio
- rapidjson
description: GNURadio blocks to read, write, and modify SigMF datasets
gitbranch: master
inherit: cmake
source: git+https://github.com/skysafe/gr-sigmf.git

This recipe is using the master branch, because your releases from the changelog are not tagged in the GitHub repo

[mattes-bru]

You'll also need a recipe for rapidjson to fulfill the dependency:

category: baseline 
description: fast JSON parser/generator for C++ with SAX/DOM style API 
satisfy:
  deb: rapidjson-dev
inherit: cmake
source: git+https://github.com/Tencent/rapidjson.git
gitrev: tags/v1.1.0