Add shortcut to modify output of a system instance

[alx87grd]

Find a way to have this behaviour:

input

sys = instance of an object where the y output is a px1 vector

output

single_output_sys = indentical instance with the exception that the y output is a sub-set (simplest case: one element) of the original output

ideal behaviour

single_output_sys = sys.set_output_to( i = 2 )

this would return a new instance indentical except that the output signal is a 1x1 vector. that correspond to the i element of the output of the original system.