Add support for Greybox model DOF counitng in degrees_of_freedom function #7206

Job	Run time
Check Spelling	6s
Compatibility tests	3m 57s
Check code formatting (Black)	31s
Pylint	4m 17s
pytest (py3.9/linux)	20m 31s
Build Sphinx docs	6m 1s
pytest (py3.9/win64)	30m 27s
pytest (py3.10/linux)	13m 30s
pytest (py3.10/win64)	30m 9s
pytest (py3.11/linux)	19m 32s
pytest (py3.11/win64)	38m 52s
pytest (py3.12/linux)	12m 45s
pytest (py3.12/win64)	29m 5s
Upload coverage report (Codecov) (linux)	10s
Upload coverage report (Codecov) (win64)	9s
	3h 30m 2s

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