Microgrid heating

[vtnate]

Any background context you want to provide?

Microgrids can include heating as well as electrical components. This adds a simple boiler to the microgrid project.

What does this PR accomplish?

• New simple boiler template
• Code to render the template using sys-params
• A test to confirm that the model is generated and simulates successfully
• Use more realistic values for boiler massflow rates in test sys-param files

  How should this be manually tested?

  CI is sufficient, IMHO