Connectors should be consolidated

[At11011]

Currently, connector.rs as implementations for a mass connector MConnector and an energy connector EConnector. I think we should consolidate these into a single struct. Energy and mass are linked, in the sense that mass itself has inherent energy. Because of this I believe they should be consolidated. The following can change:

• Instead of two separate connector types with a mass and an energy field, there should be a single connector with both fields. The energy can be calculated on-the-fly using thermodynamics during the simulation. That way, energy can be carried with mass.
• A special type of connector that supplies unusual forms of energy (such as electrical energy) can be added separately, but it is important for the sake of trustworthy energy balances, for connectors to have both energy and mass as fields that can be tracked.

I believe this will make performing energy balances go by more smoothly as explained in #19 .

[bhargavakula01]

I think there might be a situation where you want to model energy going into or out of a system, and there might not be any substance flowing there. This could be implemented through a flag stating whether a stream is an energy stream. If it is an energy stream then maybe the thermodynamics object could be null for that energy stream because the energy flow within that stream will depend on the calculations conducted in the block that the energy stream is leaving (or if it isn't coming from any block, the user would need to specify the energy flow content of that stream.

Lemme know if that makes sense @At11011