Added a Logger to Sigma, that can outputs to std::cout or any other ...

...output stream. Also there a little addition in CMakeFileLists.txt to allow to generate a executable for each one .cpp file in src/tests/

This should unify all output stuff and avoid something like It's said in Issue #93

An example of usage : 

    int main () {
        std::ofstream f("log.txt", std::ios::app);
        Log::Init(f);
        ...
        LOG_DEBUG << "Hello world!"; // Appends to log.txt -> [DEBUG] Hello world!
        LOG << "Hello world!"; // [INFO] Hello world!
        LOG_WARN << "Hello world!"; // [WARNING] Hello world!
        LOG_ERROR << "Hello world!"; // [ERROR] Hello world!
        ...
        Log::Level(Log::LogLevel::WARN); // Change filtering level
        LOG_DEBUG << "Hello world! 2"; // This is not logged
        LOG << "Hello world! 2"; // Idem
        LOG_WARN << "Hello world! 2"; // But this yes as <= that the Log Level
        LOG_ERROR << "Hello world! 2"; // Idem
        ...
    } // RAII closes the log.txt file