Fix mv read loop hardcoding 4 instead of nnode

The pb loop just above correctly uses nnode for both the loop bound
and file number offset. The mv loop should match.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/pre_process/m_start_up.fpp

@@ -476,10 +476,10 @@ contains
             end do
 
             do i = 1, nb
-                do r = 1, 4
+                do r = 1, nnode
                     ! Checking whether data file associated with variable position
                     ! of the currently manipulated bubble variable exists
-                    write (file_num, '(I0)') sys_size + r + (i - 1)*4
+                    write (file_num, '(I0)') sys_size + r + (i - 1)*nnode
                     file_loc = trim(t_step_dir)//'/mv'// &
                                trim(file_num)//'.dat'
                     inquire (FILE=trim(file_loc), EXIST=file_check)