Order Ports by order in which computed

[zenna]

Given a parametric inverse f, find an ordering of the ports, order its Ports by the order in which they are computed.

"""
# Arguments:
- `prts`: Ports from the same arrow
# Returns:
- `ordering`: `prts[ordering[t]]` is the port in ordering computed. 
function order_ports(prts::vector{Port})::Vector{Int}

This function is currently needed for analysis. We want to be able to see how the node errors of primitives vary across the arrow as a function of the position in which it is computed (e.g. to determine if solving earlier constraints first is easier), and how this varies as a function of the optimization process.