* by default no metadata is allowed and the library behaves as it already did
* if a user explicitly adds metadata (last template parameter) each node has a data member of this type and access functions for this member
* add test for this feature
* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file