improve memory efficiency of reduceByKey at line 363

[rokroskar]

The reduceByKey uses only the ghost particles but maybe all of them are kept in memory because of the slice in getting no_ghosts_rdd.

Could try something like

def get_ghosts(p): 
   ghosts = p[np.where(p['is_ghost'] != GHOST_PARTICLE_COPY)[0]]
   del(p)
   return ghosts

Has to be tested if this actually reduces the memory or not.