Two ways to define objectives, should be one

[explodingdinosaurs]

Currently, there are two way to define objectives:

1. Through association of an objective with a port, then adding it to the ObjectiveSet.objective_list attribute
2. Through setting port.objective

When optimiser.optimise is called, it calls optimiser.build_model_and_objective, which calls optimiser._build_objective which collects all the objectives from the ObjectiveSet as well as scans across all ports in the graph to see if there's objectives in port.objective.

Maybe there is a reason for this but @explodingdinosaurs and @mikejturner couldn't find one. We will need to have a conversation with @annaskobeleva and probably @joshvote to make a decision about what to do.