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

include/mujoco/mjmodel.h

@@ -194,6 +194,7 @@ typedef enum mjtDyn_ {            // type of actuator dynamics
   mjDYN_NONE          = 0,        // no internal dynamics; ctrl specifies force
   mjDYN_INTEGRATOR,               // integrator: da/dt = u
   mjDYN_FILTER,                   // linear filter: da/dt = (u-a) / tau
+  mjDYN_FILTEREXACT,              // linear filter: da/dt = (u-a) / tau, with exact integration
   mjDYN_MUSCLE,                   // piece-wise linear filter with two time constants
   mjDYN_USER                      // user-defined dynamics type
 } mjtDyn;
@@ -903,6 +904,7 @@ struct mjModel_ {
   mjtNum*   actuator_dynprm;      // dynamics parameters                      (nu x mjNDYN)
   mjtNum*   actuator_gainprm;     // gain parameters                          (nu x mjNGAIN)
   mjtNum*   actuator_biasprm;     // bias parameters                          (nu x mjNBIAS)
+  mjtByte*  actuator_actearly;    // step activation before force             (nu x 1)
   mjtNum*   actuator_ctrlrange;   // range of controls                        (nu x 2)
   mjtNum*   actuator_forcerange;  // range of forces                          (nu x 2)
   mjtNum*   actuator_actrange;    // range of activations                     (nu x 2)

include/mujoco/mjxmacro.h

@@ -387,6 +387,7 @@
     X   ( mjtNum,  actuator_dynprm,       nu,            mjNDYN               ) \
     X   ( mjtNum,  actuator_gainprm,      nu,            mjNGAIN              ) \
     X   ( mjtNum,  actuator_biasprm,      nu,            mjNBIAS              ) \
+    X   ( mjtByte, actuator_actearly,     nu,            1                    ) \
     XMJV( mjtNum,  actuator_ctrlrange,    nu,            2                    ) \
     X   ( mjtNum,  actuator_forcerange,   nu,            2                    ) \
     XMJV( mjtNum,  actuator_actrange,     nu,            2                    ) \