Oncosts tool doesn't display salary exchange figures correctly

The oncosts tool is not calculating employee pension contributions as salary exchange for exchange scheme options.

In these scenarios, employee contributions should be displayed as an "Exchange" column (as per https://www.hr.admin.cam.ac.uk/Salaries/242 when an Exchange scheme option is selected). These should then be deducted from the base salaries in order to calculate the NI and Apprenticeship Levy figures.