Updates classic cost model to append new response costs instead of overwriting them.

[metroid-samus]

This PR adds the following changes:

• Adds the incident review cost to the incident only when the incident review document is created.
• Updates the classic cost model to add new incident response costs since the last update instead of recalculating the entire cost.
• Refactors the Incident Cost service.
• Increases code coverage of the Incident Cost service to 90%.
• Improves documentation.