FABM 2.1.4

This maintenance release:

• improves pyfabm inline documentation and type annotations
• adds an example notebook that shows how to vary parameter values from Python
• enables built-in interior_temporal_mean to operate on state variables
• fixes a bug that broke single-term weighted_sum instances set in fabm.yaml
• adds standard variables for number_of_days_in_year and maximum_time_step