Poor-man pw.x parallel launcher. Usage (for mpirun):
    mpirun -np Np manypw.x -ni Ni [other options]
 or whatever is appropriate for your parallel environment

 Starts Ni pw.x instances each running on Np/Ni processors

 Each pw.x instances:

 * reads input data from from pw_N.in, N=0,..,,Ni-1 if no input
   file is specified via the -i option; from "input_file"_N
   if command-line options -i "input_file" is specified

 * You can change the number of the first index with the
   environment variable FIRST_IMAGE_INDEX, i.e. if you set
   export FIRST_IMAGE_INDEX=N_first
   then images will go from N_first N_first+Ni-1

 * saves temporary and final data to "outdir"_N/ directory
   (or to tmp_N/ if outdir='./')

 * writes output to pw_N.out in the current directory if no input
   file is specified via the -i option; to "input_file"_N.out
   if command-line options -i "input_file" is specified