Add support for Greybox model DOF counitng in degrees_of_freedom function #7079

Job	Run time
Compatibility tests	4m 22s
Check code formatting (Black)	30s
Check Spelling	8s
Pylint	4m 4s
Build Sphinx docs	5m 7s
pytest (py3.9/linux)	20m 49s
pytest (py3.9/win64)	26m 51s
pytest (py3.10/linux)	13m 32s
pytest (py3.10/win64)	31m 50s
pytest (py3.11/linux)	19m 45s
pytest (py3.11/win64)	40m 40s
pytest (py3.12/linux)	12m 9s
pytest (py3.12/win64)	29m 57s
Upload coverage report (Codecov) (linux)	9s
Upload coverage report (Codecov) (win64)	21s
	3h 30m 14s

Provide feedback