Improved behaviour of mouse-spring interactions.

Also:

- Renamed recently introduced `mjvPerturb` member `reflocalpos` -> `refselpos` and corrected docstring.
- Added `mjvPerturb.localmass`: body spatial inertia at selection point.
- Removed `const` qualifier to `mjData` argument of `mjv_initPerturb()`, to facilitate spatial inertia computation.
- Improved `mj_Object{Velocity, Acceleration}` docstrings, clarifying that rotational velocities precede linear velocities.
- Removed redundant const qualifiers from non pointer arguments to `mju_dot` and `mju_writeNumBytes`.

PiperOrigin-RevId: 511002168
Change-Id: I27400f497947f330072d4abb7c9bf7e71e6c5d49

src/engine/engine_util_misc.c

@@ -947,7 +947,7 @@ int mju_str2Type(const char* str) {
 
 
 // return human readable number of bytes using standard letter suffix
-const char* mju_writeNumBytes(const size_t nbytes) {
+const char* mju_writeNumBytes(size_t nbytes) {
   int i;
   static mjTHREADLOCAL char message[20];
   static const char suffix[] = " KMGTPE";