Add support for Greybox model DOF counitng in degrees_of_freedom function #7218

Job	Run time
Compatibility tests	4m 40s
Check code formatting (Black)	33s
Check Spelling	8s
Pylint	4m 32s
Build Sphinx docs	5m 57s
pytest (py3.9/linux)	19m 43s
pytest (py3.9/win64)	34m 17s
pytest (py3.10/linux)	13m 30s
pytest (py3.10/win64)	33m 27s
pytest (py3.11/linux)	20m 8s
pytest (py3.11/win64)	42m 8s
pytest (py3.12/linux)	12m 54s
pytest (py3.12/win64)	26m 31s
Upload coverage report (Codecov) (linux)	14s
Upload coverage report (Codecov) (win64)	10s
	3h 38m 52s

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