Set up the irrigation module demand and realized exchange

[SouthEndMusic]

Add to the irrigation module API (see also the prototype in #3148):

• get_demand!: The irrigation module computes the demands based on the soil state and returns the demands vector
• set_allocated!: The irrigation module takes in flows that are used when updating the soil model. Note: what is called realized in Ribasim is called allocated in Wflow, because Wflow doesn't have the allocated vs realized distinction.

At this point we can add integration tests like run_irrigation_standalone.jl in the prototype.

Also add the relevant irrigation module API to the developer docs.