Implement start of musculotendon model in sympy.physics._biomechanics

[brocksam]

This PR follows directly on from #25553 and should not be merged until #25553 has been merged.

References to other Issues or PRs

Forms part of the CZI biomechanics work program described in #24240. Follows on from #25553.

Brief description of what is fixed or changed

Introduces a new class, MusculotendonDeGroote2016, which implements musculotendon dynamics as a subclass of sympy.physics.mechanics.actuator.ForceActuator.

This PR introduces two formulations for musculotendon dynamics:
0. Rigid tendon.

1. Explicit elastic tendon (using muscle fiber length as the state variable).

Boilerplate is also added to support other musculotendon dynamics formulations in the future:
2. Explicit elastic tendon (using tendon force as the state variable).
3. Implicit elastic tendon (using muscle fiber length as the state variable and muscle fiber velocity as the additional input variable).
4. Implicit elastic tendon (using tendon force as the state variable and muscle fiber velocity as the additional input variable).

Other comments

This PR is pretty rough and ready as is mean to just get some form of musculotendon dynamics into the WIP sympy.physics_biomechanics module. Other PRs this month will add additional tests, complete docstrings etc.

Release Notes

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**_This PR follows directly on from #25553 and should not be merged until #25553 has been merged._**

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#### References to other Issues or PRs
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Forms part of the CZI biomechanics work program described in https://github.com/sympy/sympy/issues/24240. Follows on from #25553.

#### Brief description of what is fixed or changed

Introduces a new class, `MusculotendonDeGroote2016`, which implements musculotendon dynamics as a subclass of `sympy.physics.mechanics.actuator.ForceActuator`.

This PR introduces two formulations for musculotendon dynamics:
0. Rigid tendon.
1. Explicit elastic tendon (using muscle fiber length as the state variable).

Boilerplate is also added to support other musculotendon dynamics formulations in the future:
2. Explicit elastic tendon (using tendon force as the state variable).
3. Implicit elastic tendon (using muscle fiber length as the state variable and muscle fiber velocity as the additional input variable).
4. Implicit elastic tendon (using tendon force as the state variable and muscle fiber velocity as the additional input variable).

#### Other comments

This PR is pretty rough and ready as is mean to just get some form of musculotendon dynamics into the WIP `sympy.physics_biomechanics` module. Other PRs this month will add additional tests, complete docstrings etc.

#### Release Notes

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The following commits add new files:

• 312f91f:
  • sympy/physics/_biomechanics/tests/test_musculotendon.py
• ee8c45d:
  • sympy/physics/_biomechanics/musculotendon.py

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[moorepants]

LGTM, merging as private so we can get the full examples working. Will review in detail as we build out the examples and before we make _biomechanics public.