Before this patch, `basic_json` types with different template arguments
were treated as `CompatibleArrayType`. Which sometimes leads to recursive
calls and stack overflows.
This patch adds a constructor and a `get` overload to deal with
different `basic_json` types.
Beside the fix discussed in #961, we also had to re-adjust a test case. It seems that it was failing before, and I "fixed" it to work with the broken implementation...
* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file
