Combines computing value and derivative of muscle curves in Millard

This PR replaces separate calls to calcValue and calcDerivative on the muscle curves in Millard2012EquilibriumMuscle by calcValueAndDerivative, which is more efficient.

The MuscleLengthInfo::userDefinedLengthExtras and FiberVelocityInfo::userDefinedVelocityExtras buffers are used to store the derivatives of the curves until needed.

Measuring the wallclock time of Rajagopal forward integration, and CMC running-model (using Millard muscles) shows around 10% perf benefit:

	main	with this PR
CMC-Running Model	25.8 [s]	22.9 [s] (-12.4%)
Rajagopal Forward Integration	6.4 [s]	5.8 [s] ( -9.8% )

Brief summary of changes

There are 4 relevant muscle curves in Millard2012EquilibriumMuscle which are calling calcValue and calcDerivative, which is combined to calcValueAndDerivative by this PR:

Added calcValueAndDerivative method to:
- ActiveForceLengthCurve.h
- TendonForceLengthCurve.h
- FiberForceLengthCurve.h
- TendonForceLengthCurve.h
Regarding the ActiveForceLengthCurve and TendonForceLengthCurve:
- moved computing curve derivative to calcMuscleLengthInfo, and store result in MuscleLengthInfo::userDefinedLengthExtras
- moved Millard2012EquilibriumMuscle::calcFiberStiffness to anonymous namespace (this was using the derivative values)
Regarding the ForceVelocityCurve:
- use calcValueAndDerivative during calcDampedNormFiberVelocity() instead of calcValue followed by calcDerivative.
- return last evaluation of fiberForceVelocityCurve from calcDampedNormFiberVelocity()
Regarding the TendonForceLengthCurve:
- Compute fiberstiffness and tendonforcelength multiplier during calcFiberVelocityInfo() and store in FiberVelocityInfo::userDefinedVelocityExtras

Testing I've completed

Unit tests are passing, and Rajagopal forward simulation as well as CMC on running-model gives no change in results.

CHANGELOG.md

no need to update because too minor change.

This change is

opensim-org / opensim-core