Make activation, muscle length and lengthening speed variables const since algorithm must not change these.
Make activation, muscle length and lengthening speed variables const since algorithm must not change these.

opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core