Add min_site_addition_per_year parameter to downscaling #63

Job	Run time
Mamba Test	6m 59s
Pixi-Locked Test	24s
Python Tests (Pixi) (windows-latest)	10m 18s
Python Tests (Pixi) (macos-latest)	7m 54s
Python Tests (Pixi) (ubuntu-latest)	5m 56s
	31m 31s

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