Add mass-flow connections in addition to power-flow

[michael-okeefe]

Problem:
We currently have power-flow connections in ERIN. However, there is a need to track mass-flow.

Proposed Solution:
Add specific mass-flow connections and only allow components that make sense to work with mass flow to work. Mass-flow components would need the ability to track their own units and would need new component types to allow conversion from mass to powerflow (e.g., a fuel flow that goes to a converter would need a component that interprets X gallons/min of diesel fuel as a kW flow based on higher-heating value).