Add functionality to do circular calculation across unconnected unit processes

[spencer-in-github]

Situation: While trying to add a geologic hydrogen pathway to OPGEEv4.0, there is a potential case that we would use the produced crude gas (After separation and before further processing) to generate electricity for all the energy required onsite. However, to subtract the precise amount of crude gas used for onsite electricity generation after separation, it would require information on uncalculated unit processes as further processing units are not calculated yet in this step of the model run.

Task: Add a functionality where we can do circular calculation across the unit processes until numerical convergence, i.e. we can run through all unit processes first to get an idea of how much crude gas we need for onsite activities, and come back to the separation step to subtract the calculated gas amount for onsite electricity; and iterate this step until convergence.