Add self-diffusivity calculation #84

Job	Run time
environment-config	4s
black_check	10s
pylint_check	46s
pypi_check	20s
main-tests (macOS-latest, 3.9, latest)	8m 11s
main-tests (macOS-latest, 3.9, develop)	14m 1s
main-tests (macOS-latest, 3.10, latest)	9m 36s
main-tests (macOS-latest, 3.10, develop)	16m 33s
main-tests (macOS-latest, 3.11, latest)	8m 30s
main-tests (macOS-latest, 3.11, develop)	14m 13s
main-tests (ubuntu-latest, 3.9, latest)	3m 19s
main-tests (ubuntu-latest, 3.9, develop)	5m 54s
main-tests (ubuntu-latest, 3.10, latest)	3m 43s
main-tests (ubuntu-latest, 3.10, develop)	6m 47s
main-tests (ubuntu-latest, 3.11, latest)	3m 16s
main-tests (ubuntu-latest, 3.11, develop)	5m 29s
main-tests (windows-latest, 3.9, latest)	10m 11s
main-tests (windows-latest, 3.9, develop)	11m 19s
main-tests (windows-latest, 3.10, latest)	13m 20s
main-tests (windows-latest, 3.10, develop)	15m 13s
main-tests (windows-latest, 3.11, latest)	11m 33s
main-tests (windows-latest, 3.11, develop)	13m 39s
	2h 56m 7s

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