Removes musclePower field from MuscleDynamicsInfo

[pepbos]

This PR removes the musclePower field from the cached MuscleDynamicsInfo.

Since the cache's main purpose is to store expensive-to-compute results, and the muscle power is very cheap to compute, it can be removed from the cache.

Brief summary of changes

• Removed musclePower field from MuscleDynamicsInfo in Muscle.h.
• Added calcMusclePower method to Muscle in Muscle.h.
• Removed power computation in all children of Muscle.

Testing I've completed

Unit tests are passing, and contain regression tests for several muscles.

CHANGELOG.md

• no need to update because too small a change.

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This change is 

[pepbos]

@adamkewley , @tkuchida thanks for your comments! I think it is good to merge, but have a look if you have the time