Implement centralized co-optimization in Pyomo

@S.J.vanRijn Here is a simple pyomo implementation of flexibility scheduling using the centralized co-optimization approach. The sector-coupled energy system currently consists of generator agents and only one prosumer (the heat system agent). Other prosumer agents listed in the config.yaml file will be added in the next issue.

I am wondering on how we should organize the code. To perform a scalability test, we might need to add tens of generator or prosumer agents to the system with perhaps different assets (which means slightly different pyomo model). With the current implementation, I think it will be difficult. What I mean is that if we want to add another heat agent that only has a gas boiler and an e-boiler (not a HP or CHP), then the current code cannot be adapted easily (given the current pyomo model components naming style). We can think about this later before we build a large system.

Closes #7 (closed)