Ruff bump to >= 0.16.0 and major cleaning - #65

analysis/u00_regionalization.ipynb:1:1: F821 Undefined name `_polygons_impact`

analysis/u00_regionalization.ipynb:1:1: F841 Local variable `names` is assigned to but never used help: Remove assignment to unused variable `names`

analysis/u00_regionalization.ipynb:1:1: B006 Do not use mutable data structures for argument defaults help: Replace with `None`; initialize within function

analysis/diagnostic_utils.py:73:9: F841 Local variable `ref_vals` is assigned to but never used help: Remove assignment to unused variable `ref_vals`

analysis/analysis_utils.py:616:28: RUF059 Unpacked variable `step` is never used help: Prefix it with an underscore or any other dummy variable pattern

analysis/analysis_utils.py:616:11: RUF059 Unpacked variable `norm` is never used help: Prefix it with an underscore or any other dummy variable pattern

analysis/analysis_utils.py:616:5: RUF059 Unpacked variable `cmap` is never used help: Prefix it with an underscore or any other dummy variable pattern

analysis/analysis_utils.py:63:12: B006 Do not use mutable data structures for argument defaults help: Replace with `None`; initialize within function

analysis/a06_postcard_map.ipynb:1:1: F821 Undefined name `ds2`