Allow part-time staff members

[yld-weng]

Part time staff members aren't captured. Ability to add real FTE of staff members that are part-time.

Admin should also add in periods of time for the FTE of a staff member. E.g. A staff from other department will only work 50% for RSE on the system for certain periods of time:

• 01.01.2023 - 31.03.2023 50%
• 01.04.2023 - 31.05.2023 0%
  so the cost implications don't show.

TODOs:

• Give option to specify FTE when creating a new staff?
• How to implement periods of time?
• How would changes affecting calculations?