Fix usage of uninitialized variable

Description

The classical particles' acceleration is not always available, so we don't print it to the log anymore.

News snippet

Fix usage of uninitialized variable in classical particles.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

src/classical/classical_particle.F90

@@ -410,11 +410,10 @@ contains
     write(fmt,'("(4X,A,1X,",I2,"e14.6)")') this%space%dim
     write(message(2),fmt) "Coordinates: ", (this%pos(idir), idir = 1, this%space%dim)
     write(message(3),fmt) "Velocity:    ", (this%vel(idir), idir = 1, this%space%dim)
-    write(message(4),fmt) "Acceleration:", (this%acc(idir), idir = 1, this%space%dim)
-    write(message(5),fmt) "Force:       ", (this%tot_force(idir), idir = 1, this%space%dim)
-    write(message(6),'(4x,A,I8.7)')  'Clock tick:      ', this%clock%get_tick()
-    write(message(7),'(4x,A,e14.6)') 'Simulation time: ', this%clock%time()
-    call messages_info(7)
+    write(message(4),fmt) "Force:       ", (this%tot_force(idir), idir = 1, this%space%dim)
+    write(message(5),'(4x,A,I8.7)')  'Clock tick:      ', this%clock%get_tick()
+    write(message(6),'(4x,A,e14.6)') 'Simulation time: ', this%clock%time()
+    call messages_info(6)
 
     POP_SUB(classical_particle_iteration_info)
   end subroutine classical_particle_iteration_info