When a player can not play, a boolean is set, possible moves is set to 1  and the game accepts whatever value as a move.

This reverts commit a6c5137c.
The program would exit with an error.

Added a virtual loss when a thread is exploring so that all threads don't explore the same branch

TODO: implementation of every function.
connect4.cpp and test_connect4.cpp were left in the sources of the
Makefile so the project could compile