move inflow points to confluence if within buffer from model boundary

[DirkEilander]

Issue addressed

Fixes issue with multiple inflow points near confluence.
In case of a confluence near the model region I want to be able to select the confluence (see figure b) rather than both tributaries as inflow points (see figure a). This is achieved by removing small river segments that completely fall within a user-defined buffer from the model boundary. The most upstream point of the remaining river is than located at the confluence.

figure a: inflow at to closeby tributaries before fix

figure b: inflow at confluence after fix

Checklist

• Updated tests or added new tests
• Branch is up to date with main
• Updated documentation if needed
• Updated changelog.rst if needed

Additional Notes (optional)

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