Add support for Greybox model DOF counitng in degrees_of_freedom function #7215

Job	Run time
Check Spelling	10s
Compatibility tests	6s
Check code formatting (Black)	32s
Build Sphinx docs	10s
pytest (py3.9/linux)	11s
Pylint	16s
pytest (py3.9/win64)	15s
pytest (py3.10/linux)	11s
pytest (py3.10/win64)	7s
pytest (py3.11/linux)	11s
pytest (py3.11/win64)	9s
pytest (py3.12/linux)	12s
pytest (py3.12/win64)	8s
Upload coverage report (Codecov)	1s
	2m 39s

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