Added a sit-to-stand example with the gait2d model. #672

Job	Run time
Build and run tests (ubuntu-latest, 3.9)	3m 39s
Build and run tests (ubuntu-latest, 3.10)	3m 41s
Build and run tests (ubuntu-latest, 3.11)	3m 41s
Build and run tests (ubuntu-latest, 3.12)	3m 41s
Build and run tests (ubuntu-latest, 3.13)	3m 41s
Build and run tests (macos-13, 3.9)	1s
Build and run tests (macos-13, 3.10)	3m 26s
Build and run tests (macos-13, 3.11)	3m 28s
Build and run tests (macos-13, 3.12)	3m 39s
Build and run tests (macos-13, 3.13)	3m 39s
Build and run tests (macos-latest, 3.9)	1s
Build and run tests (macos-latest, 3.10)	3m 29s
Build and run tests (macos-latest, 3.11)	1s
Build and run tests (macos-latest, 3.12)	1s
Build and run tests (macos-latest, 3.13)	1s
Build and run tests (windows-latest, 3.9)	3m 33s
Build and run tests (windows-latest, 3.10)	3m 25s
Build and run tests (windows-latest, 3.11)	3m 25s
Build and run tests (windows-latest, 3.12)	3m 25s
Build and run tests (windows-latest, 3.13)	3m 24s
	53m 21s

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