Runtime disabling of actuators according to group.

Added `option-actuatorgroupdisable` attribute and associated `mjOption.disableactuator` integer bitfield, used to disable sets of actuators at runtime according to their group.

- The first 6 actuator groups are toggleable in the `simulate` viewer.
- Minor refactor and cleanup of actuator documentation.

https://youtu.be/H9qG9Zf2W44

Fixes #1092.

PiperOrigin-RevId: 578335600
Change-Id: I4cf663b90ea768e4380acfa2fe3b8c15c7cbb568

include/mujoco/mjmacro.h

@@ -34,6 +34,9 @@
 #define mjDISABLED(x) (m->opt.disableflags & (x))
 #define mjENABLED(x)  (m->opt.enableflags & (x))
 
+// is actuator disabled
+#define mjACTUATORDISABLED(i) (m->opt.disableactuator & (1 << m->actuator_group[i]))
+
 // annotation for functions that accept printf-like variadic arguments
 #ifndef mjPRINTFLIKE
   #if defined(__GNUC__)

include/mujoco/mjmodel.h

@@ -433,6 +433,7 @@ struct mjOption_ {                // physics options
   int mpr_iterations;             // maximum number of MPR solver iterations
   int disableflags;               // bit flags for disabling standard features
   int enableflags;                // bit flags for enabling optional features
+  int disableactuator;            // bit flags for disabling actuators by group id
 
   // sdf collision settings
   int sdf_initpoints;             // number of starting points for gradient descent

include/mujoco/mjxmacro.h

@@ -43,6 +43,7 @@
     X( int,     mpr_iterations    ) \
     X( int,     disableflags      ) \
     X( int,     enableflags       ) \
+    X( int,     disableactuator   ) \
     X( int,     sdf_initpoints    ) \
     X( int,     sdf_iterations    )