Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce?

Feature: this PR adds a capital cost to the supplementary duct firing system that allows CHP systems to produce more heat than that which is captured via the prime mover when generating electricity.

What is the current behavior?

• Supplementary firing is included and incurs a cost according to the thermal efficiency of the system, but no capital cost for this add-on exists.
• Supplementary firing thermal production is not included in thermal output summary.

What is the new behavior (if this is a feature change)?

• Capital cost per kW of the supplementary firing system has been added as a new input, and the size of the supplementary firing system in the solution has been added as an output.
• A new binary variable and continuous variable have been added to the optimization model; the former is 1 if supplementary firing is included with the CHP system and 0 otherwise, while the second represents the size of the supplementary firing system.
• A collection of constraints have been added so that the size of the supplementary firing system is equal to the CHP system size if any supplementary firing is used, and zero otherwise.
• Supplementary firing thermal production is combined with the CHP prime mover heat capture in tracking CHP thermal production as a combined statistic (It's not currently separated in the outputs).

Does this PR introduce a breaking change?

No

Other information:

• Capital cost calculation is included in the initial capex costing when processing outputs.
• Thermal production from supplementary firing isn't tracked separately anywhere in the Julia outputs right now.