fix cmake install directory (for real this time)

* Rename 'develop' folder to 'include/nlohmann'
* Rename 'src' folder to 'single_include/nlohmann'
* Use <nlohmann/*> headers in sources and tests
* Change amalgamate config file

test/src/unit-unicode.cpp

@@ -29,7 +29,7 @@ SOFTWARE.
 #include "catch.hpp"
 
 #define private public
-#include "json.hpp"
+#include <nlohmann/json.hpp>
 using nlohmann::json;
 
 #include <fstream>