Python "thread" Pool is not really thread safe

[TanguyUrvoy]

Sometimes the program will hang with this nasty comment:

Exception RuntimeError: RuntimeError('main thread is not in main loop',) in <bound method PhotoImage.__del__ of <Tkinter.PhotoImage instance at 0x7f58021bc6c8>> ignored Tcl_AsyncDelete: async handler deleted by the wrong thread

A safe, but slow, workaround is to use eval_policy(nbReps, horizon, pm_game, policy) instead of
eval_policy_parallel(nbCores, nbReps, horizon, pm_game, policy).

Tanguy