Allocate to UserDemand from directly connected basin if possible

[SouthEndMusic]

Fixes #1545

Some notes on this PR:

• This still only works when the Basin upstream of a UserDemand has a level demand to define the source capacity of the basin
• It makes sense that this flow from the Basin to the UserDemand also shows up in allocation_flow.arrow, which requires some refactoring. That is fine though, as that refactor is needed anyway for Optimize allocation per source in a fixed order #565, where we probably want to aggregate the allocation flows for output over the optimizations for all sources in stead of saving them per source
• I'm not sure how to directly test this

[visr]

This still only works when the Basin upstream of a UserDemand has a level demand to define the source capacity of the basin

Link to discussion: #1428 (comment)

We should directly test this since it will have a large impact on results. Would a UserDemand - Basin - LevelDemand model suffice, with only initial storage?

This behavior probably also needs to be in the user docs, probably https://deltares.github.io/Ribasim/reference/node/user-demand.html, maybe https://deltares.github.io/Ribasim/concept/allocation.html.