inittialise a persistent globally shared memory
[sim] = ld_global_memory(sim, events, ident_str, datatype, len, initial_data, visibility, useMutex)
ident_str (string) - name of the memory
datatype - ORTD datatype of the memory (for now only ORTD.DATATYPE_FLOAT works)
len (integer) - number of elements
initial_data - initial data of the memory
visibility (string) - 'global', ... (more are following)
useMutex (integer) - 0 or 1. Use a mutex if you access the memory from different threads