Save separate files for boundary particles with different kent

To this aim we need to
- Still count the fluid parts, which are saved separately
- Count the number of particles for each kent
- Allocate one array of OutBuf for each kent
- Copy the particles to the proper array
- Save the particles in dedicate per-kent files, skipping fluid
  (which is saved separately)
Fix: don't know why, had to move num_parts_per_kent[] increment for be
Handle possibility that num parts with certain kent is 0