Add filterexact and actearly options to MuJoCo.

When filterexact is a new dyntype which is just like the existing `filter`, but the activation state is integrated with exact integration, instead of simple Euler integration.

When actearly is specified on a general actuator, `qfrc_actuator` is computed using the next timestep's `act` value, instead of the current value.

PiperOrigin-RevId: 554438423
Change-Id: If901e4988fa6b518d6f3097f149770665a189a4b

src/user/user_objects.h

@@ -1026,6 +1026,7 @@ class mjCActuator : public mjCBase {
   double dynprm[mjNDYN];          // dynamics parameters
   double gainprm[mjNGAIN];        // gain parameters
   double biasprm[mjNGAIN];        // bias parameters
+  bool actearly = false;          // apply activations to qfrc instantly
   double ctrlrange[2];            // control range
   double forcerange[2];           // force range
   double actrange[2];             // activation range