RigidTendonMuscle fibervelocity computation lacks pennation angle effect

[pepbos]

The fiberVelocity computation in RigidTendonMuscle does not take the pennation angular velocity into account. It appears to be correct only if the pennation angle is zero, which is not generally the case.

The fibervelocity computed by the RigidTendonMuscle is set to equal the muscle lengtheningspeed:

opensim-core/OpenSim/Actuators/RigidTendonMuscle.cpp

Line 164 in 20752f3

fvi.fiberVelocity = getPath().getLengtheningSpeed(s);

This would be correct if the pennation angle is zero. But the pennation angle is generally not zero:

opensim-core/OpenSim/Actuators/RigidTendonMuscle.cpp

Line 138 in 20752f3

mli.pennationAngle = acos(mli.cosPennationAngle);

It looks like the correct fibervelocity calculation can be found in Millard2012EquilibriumMuscle, with the ignore_tendon_compliance set to true:
https://github.com/opensim-org/opensim-core/blob/20752f3a7852854a3e95a0bf52832bc8ec788163/OpenSim/Actuators/Millard2012EquilibriumMuscle.cpp#L996C32-L996C32