add co-benefits calculation

[panosatha]

Kind of request

Adding new functionality

Enhancement Description

Add calculation of co-benefits in the benefits object methods when metrics table implementation is finished.

Task list

• [ ] New method in Benefit class to caclualte co-benefits
• [ ] Add unit tests

Use case

No response

Additional Context

No response

[kathrynroscoe]

This needs a discussion if we are going to keep this in. Let's get input from stakeholders in March and then make a decision.