Use x-axis along the particle's quaternion.

This change allows the fix to work in 2D as well.

doc/src/fix_propel_self.txt

@@ -40,8 +40,8 @@ which the force is added depends on the mode.
 For mode = velocity, the active force acts along the velocity vector of each atom. This can
 be interpreted as a velocity-dependent friction, such as proposed by "(Erdmann)"_#Erdmann.
 
-For mode = quaternion the force is along the axis obtained by rotating the z-axis along the
-atom's quaternion. In other words, the force is along the z-axis in the atom's body
+For mode = quaternion the force is along the axis obtained by rotating the x-axis along the
+atom's quaternion. In other words, the force is along the x-axis in the atom's body
 frame. This mode requires all atoms in the group to have a quaternion, so atom_style should
 either be ellipsoid or body.  In combination with Langevin thermostat for translation and
 rotation in the overdamped regime, the quaternion mode corresponds to the active Brownian

src/USER-MISC/fix_propel_self.cpp

@@ -188,7 +188,7 @@ void FixPropelSelf::post_force_quaternion(int /* vflag */ )
         continue;
       }
       
-      double f_act[3] = { 0.0, 0.0, 1.0 };
+      double f_act[3] = { 1.0, 0.0, 0.0 };
       double f_rot[3];
 
       double *quat  = bonus[ellipsoid[i]].quat;