[RTR] Holonomic muscle model

[p-shg]

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Working on Muscle driven dynamics for models with holonomic constraints
@pariterre the arm26_pendulum_swingup_Muscle_algebraic.py file reproduces the plotting error I told you about

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This change is 

[Ipuch]

@Ipuch reviewed 18 of 18 files at r1, all commit messages.
Reviewable status: all files reviewed, 5 unresolved discussions (waiting on @p-shg)

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bioptim/examples/holonomic_constraints/arm26_pendulum_swingup_Muscle_algebraic.py line 1 at r1 (raw file):

"""

"muscle_algebraic.py"

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bioptim/examples/holonomic_constraints/four_bar.py line 1 at r1 (raw file):

"""

add the test somewhere.

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bioptim/examples/holonomic_constraints/custom_dynamics.py line 166 at r1 (raw file):

class HolonomicMusclesDynamics(HolonomicTorqueDynamics):

put everything in custom_dynamics_muscle.py, copy paste or import from the other file.

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bioptim/examples/holonomic_constraints/arm26_pendulum_swingup.py line 1 at r1 (raw file):

"""

Not necessary

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bioptim/examples/holonomic_constraints/arm26_pendulum_swingup_Muscle.py line 1 at r1 (raw file):

"""

name of the file "_muscle.py"