feat: enables reading per degree impairment from xls input #246

Job	Run time
Tests on other platforms (windows-2019, 3.10)	4m 50s
Tests on paywalled platforms (macos-13-xlarge, 3.12)	1m 50s
Tox test (py38)	3m 38s
Tests on other platforms (windows-2022, 3.11)	4m 4s
Tox test (py39)	3m 34s
Tests on other platforms (windows-2022, 3.12)	4m 35s
Tox test (py310)	3m 42s
Tests on other platforms (macos-12, 3.11)	1s
Tox test (py311)	3m 27s
Tests on other platforms (macos-13, 3.12)	5m 25s
Tox test (py312-cover)	6m 43s
Tox test (docs, graphviz)	1m 50s
Docker image	54s
PyPI packaging	0s
	44m 33s

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