This PR interfaces the json library for C++.
It adds the module gsJSON, which can be enabled via CMake

cmake . -DGISMO_OPTIONAL="<other modules>;gsJSON"

As shown in the example json_test.cpp, the class is easily used to generate JSON files.

For example, for writing a file, we can do:

gsJSON j;
j["a"] = 2;
j["b"] = 3.0;
j["c"]["d"] = "four";
j["e"] = {"e1", "e2", "e3"};
j["kv"]=gsKnotVector<>(0,1,0,2);
j.save("file.json");

Generating the following file:

{
    "a": 2,
    "b": 3.0,
    "c": {
        "d": "four"
    },
    "e": [
        "e1",
        "e2",
        "e3"
    ],
    "kv": {
        "degree": 1,
        "knots": [
            0.0,
            0.0,
            1.0,
            1.0
        ]
    }

And for reading a file, we can do:

gsJSON j("file.json");
index_t a = j["a"].get<index_t>();
real_t b = j["b"].get<real_t>();
std::string c = j["c"]["d"].get<std::string>();
std::vector<std::string> e = j["e"].get<std::vector<std::string> >();
gsKnotVector<> kv = j["kv"].get<gsKnotVector<>>();

The commit description must be structured as a list follows:

NEW:
Add JSON support

IMPROVED:

FIXED:

API:

Please consider the following checklist before issuing a pull
request:

• Have you added an explanation of what your changes do and why
  you'd like us to include them?
• Have you documented any new codes using Doxygen comments?
• Have you written new tests or examples for your changes?