JSON replacement in library (a few toolkits)

[guruofquality]

To get packaged into debian/ubuntu need to drop the dependency on Poco::JSON specifically. The other libs are fine. There is a very clean JSON implementation for C++ and its only a single header file, which is easier than adding another library dependency. The resulting code using the API is also much more readable vs Poco JSON https://github.com/nlohmann/json

This is mostly a transparent change because most of the JSON usage is exposed as serialized strings and not objects of a specific API. Also the PothosGUI was already ported over to use the Qt JSON API. However, the device info used by some of the toolkits was exposing a Poco::JSON pointer as a return value. This will be changed to a serialized string as well, and the toolkits will depend on version 0.6 of the library

These are a list of tasks in the works so we dont forget:

• port core library to json.hpp
• an easy download option for json.hpp when missing (might also use for PothosSDR) The nlohmann/json git repo is very large so this would be easier than a submodule clone

Toolkits with JSON overlays and or device info:

• libIIO
• SDR
• gr-pothos (unit tests)
• Audio
• blocks toolkit using json for unit testing
• comms toolkit using json for unit testing
• python toolkit using json for unit testing
• opencl toolkit using json for unit testing

Basic tasks for toolkits:

• make toolkits depend on 0.6 of the library
• look for json hpp in cmake
• include json hpp in cmake
• debian/control nlohmann-json-dev
• changelog entry
• unit tests for testplan need to pass string into feeder
• device info needs to return string
• test plans need to use json hpp
• device into needs to use json hpp
• overlays need to use json hpp

Ubuntu tasks:

• Backport https://packages.ubuntu.com/yakkety/nlohmann-json-dev for xenial
• add this package to apt-get install build guide instructions
• Toolkits and library need to depend on nlohmann-json-dev for debian/control

Homebrew:

• depend on nlohmann/json/nlohmann_json (full tap and recipe path)

PothosSDR:

• nlohmann/json git seems to be large to just download the header with cmake and install (using an easy download option in core library)

[guruofquality]

A blog on this for those interested: http://www.joshknows.com/blog/79/TheEvilJsonPackagingProblem