A simple thread group

The user instanciates a ThreadGroup parameterized to process a maximum number of threads at a time.
The user then adds to the ThreadGroup functions and their arguments.
This add method blocks until there is room to launch the function in a thread.

TODO

• void add(void f(void *), void * arg) is not user friendly, use some std::function and templates
• measure the performance
• fix Makefile so main.cpp is rebuilt when ThreadGroup changes
• maybe a non-blocking add: enqueue
• use a condition variable instead of iterating through all threads for clean up