Added optional smoothing to muscle dynamics:

- `muscle` actuators take a new `tausmooth` attribute (defaults to 0) which when positive, smooths the transition between activation and deactivation timescales.
- `mju_muscleDynamics` takes 3 parameters, adding width of smoothing sigmoid.

PiperOrigin-RevId: 531473354
Change-Id: I8ab6c0289ff04437e798c3ca6edda132a30fcc31

introspect/functions.py

@@ -7294,11 +7294,11 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
                  name='prm',
                  type=ArrayType(
                      inner_type=ValueType(name='mjtNum', is_const=True),
-                     extents=(2,),
+                     extents=(3,),
                  ),
              ),
          ),
-         doc='Muscle activation dynamics, prm = (tau_act, tau_deact).',
+         doc='Muscle activation dynamics, prm = (tau_act, tau_deact, smoothing_width).',  # pylint: disable=line-too-long
      )),
     ('mju_encodePyramid',
      FunctionDecl(