Add building-side fluid networks for 1GDH

[khinkelman]

The current 1GDH model assumes ideal heat transfer at the energy transfer stations with the building's load Q_flow directly impacting the district water condensation process. Thus, the fluid network for the building-side is not currently modeled. A steam shell-and-tube heat exchanger model should be developed, the 1GDH ETS model extended, and simple building model implemented to better represent the fluid & thermal dynamics.

• Steam shell-and-tube heat exchanger
• 1GDH ETS model with district & building fluid streams
• Time series coupled building model with 1GDH ETS and simple building