inittialise a new ringbuffer
[sim] = ld_ringbuf(sim, events, ident_str, datatype, len, visibility)
ident_str (string) - name of the ringbuffer
datatype - ORTD datatype of the ringbuffer (for now only ORTD.DATATYPE_FLOAT)
len (integer) - number of elements
initial_data - initial data of the ringbuffer
visibility (string) - 'global', ... (more are following)