Implement a fuller range of on-cost commitment calculations

Created by: rjw57

We would like to be able to calculate on-cost commitments for staff members. Given the following input:

• Current grade and point
• Anniversary date
• Contract end date
• Pension scheme
• 1st August negotiated salary increment which is modelled as 2%
• Any additional (salary change, date) pairs which may happen due to, e.g., change of working hours/re-grading of role, etc.
• (Implicitly) that they are currently employed

Provide an API to calculate the total expected commitment from now until the end date of the contract. The repo has some documentation on how commitments may be calculated given the information above.

Note that the repo currently does not have the grade and salary point table available and that it should correctly handle contribution points and expected promotion dates where grade (but not necessarily point) will change.