usefull_functions/print_stack_trace: move include of stdlib and stdio outside of the #ifdef since they are useful for other functions

Modify way of writing buffer at end of simulation to make callable from cython interface (this enable writing buffer before deleting the MbsDirdyn object)

Add pre-processor flags around function point and user/symbolic lib structure declaration to avoid conflict in case of monolithic build (see #50)

Remove the initialization of user_IO to 0 from the user_IO.c file and implement it in mbs_load_xml since it is generic (the function mbs_set_user_IO should be move in another file)