Now only uses execution contexts, no more sub interpreter.
+ reviewed PyInterp_Interp class and Python initialization
+ removed GetLockWrapper() method from PyInterp_Interp class.
+ interpreter member is no more requested in PyLock_Request (can be set to NULL)
+ reviewed PyLockWrapper class design to forbid copy ctor and assignment
+ more sophisticated stdout/stderr redirection for Python command: restore what was previously used after Python command execution.
+ get rid of "shared_modules" logic - no more necessary with a single interpreter
