Include a third-party JSON library

[ylee88]

nlohmann/json is a header-only library, so it can be included in this repo as a third party. This PR will let the user build Milhoja without cloning nlohmann/json by themself.

[kweide]

Using this file json.hpp is INSANE overhead... ~ 920,000 bytes in 24765 source lines, just to write one JSON file... Maybe I am missing something, but the only real use of this header file appears to be in tools/createSizesJson.cpp, for writing a few lines to the file sizes.json. (That and some use for testing, see test/RuntimeParameters.h).

Moreover, the file tools/createSizesJson.cpp already has code for writing sizes.json, it is just commented out! See

        //output << "{" << std::endl;                                                                                                                       

        // TODO: There should be a better way to print the name of a data                                                                                   
        //       type and its size by using a std::tuple object as well as                                                                                  
        //       typeid(T).name(). This way we can create a tuple object filled                                                                             
        //       with types, making it easy to loop over as well as add new                                                                                 
        //       types if necessary. Also, GCC would not need a special env                                                                                 
        //       variable for using a specific json library. However, for the                                                                               
        //       sake of getting the datapacket_tests into main, I'll ignore                                                                                
        //       this for now.                                                                                                                              
        //output << "    \"byte_align\": " << BYTE_ALIGNMENT << "," << std::endl;                                                                           
        //output << "    \"int\": " << sizeof(int) << "," << std::endl;                                                                                     
        //output << "    \"unsigned int\": " << sizeof(unsigned int) << "," << std::endl;                                                                   
        //output << "    \"std::size_t\": " << sizeof(std::size_t) << "," << std::endl;                                                                     
        //output << "    \"real\": " << sizeof(milhoja::Real) << "," << std::endl;                                                                          
        //output << "    \"IntVect\": " << sizeof(milhoja::IntVect) << "," << std::endl;                                                                    
	//output << "    \"RealVect\": " << sizeof(milhoja::RealVect) << "," << std::endl;                                                                  
	//output << "    \"FArray1D\": " << sizeof(milhoja::FArray1D) << "," << std::endl;                                                                  
	//output << "    \"FArray2D\": " << sizeof(milhoja::FArray2D) << "," << std::endl;                                                                  
	//output << "    \"FArray3D\": " << sizeof(milhoja::FArray3D) << "," << std::endl;                                                                  
	//output << "    \"FArray4D\": " << sizeof(milhoja::FArray4D) << std::endl;                                                                         
	output << sizes.dump(INDENT);

	//output << "}" << std::endl;                                                                                                                       

All it should take is uncommenting the commented code lines, and vice versa, in the above code snippet - and removing the include. (Untested)

I am willing to create an alternative PR tomorrow to get rid of this dependency on json.hpp.

[ylee88]

I don't think it is a big overhead; as you already pointed out, the only file has #include <nlohmann/json.hpp> line is just tools/createSizesJson.cpp. So this giant header file is only included in compiling tools/createSizesJson.cpp.

But I agree that using an external library just to write a simple JSON file may be an overkill. I think Jaread wanted to make this more extendable by using a well-known library. I prefer to use an existing library instead of writing some logic ourselves.