Benchmark of Json implementations

[prudhomm]

Feelpp use the json format to describe the physical models for the toolboxes and setup configuration for various applications
We use two implementations nlohmann/json and boost::property_tree. The latter initially as a convenience because we use Boost and the former because it provides a much more powerful and feature complete API for json.

The results of running the tests are here:

https://lokiastari.com/Json/Performance.osx.html
https://lokiastari.com/Json/Performance.linux.html
Here is a benchmark discussion on existing implementations of json libraries
nlohmann/json#2309 (comment)