ENH: Fix `MPI` class RecursionError w multiprocessing

Hi, the use of the multiprocessing module with NEBs leads to RecursionError, as shown in the minimal example below. Using pickle on NEBs also gives the same error - I added another minimal example below for this. These problems when using multiprocessing and pickle seems to be due to __getattr__ being called on an empty instance of MPI, see for example, https://stackoverflow.com/a/34950256 and https://stackoverflow.com/a/50888571.

The RecursionError can be simply solved by checking if __getattr__ is trying to access the __setstate__ attribute, and raising an AttributeError if so. This merge request implements this fix.

Thanks for considering this!

Minimal example - RecursionError with multiprocessing

#!/usr/bin/env python

from ase import Atoms
from ase.neb import NEB
from multiprocessing import Pool

def print_neb_images(neb):
    print(neb.images)

images = [Atoms('H')] * 3
nebs = [NEB(images)] * 3

with Pool(2) as p:
    p.map(print_neb_images, nebs) # raises RecursionError

Output

Process ForkPoolWorker-2:
Traceback (most recent call last):
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/multiprocessing/pool.py", line 110, in worker
    task = get()
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/multiprocessing/queues.py", line 354, in get
    return _ForkingPickler.loads(res)
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/site-packages/ase/parallel.py", line 92, in __getattr__
    if self.comm is None:
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/site-packages/ase/parallel.py", line 92, in __getattr__
    if self.comm is None:
  File "/home/benjamin/miniconda3/envs/test/lib/python3.7/site-packages/ase/parallel.py", line 92, in __getattr__
    if self.comm is None:
  [Previous line repeated 489 more times]
RecursionError: maximum recursion depth exceeded

Minimal example - RecursionError with pickle

#!/usr/bin/env python

import pickle
from ase import Atoms
from ase.neb import NEB

images = [Atoms('H')] * 3
nebs = [NEB(images)] * 3

pickled_nebs = pickle.dumps(nebs)
pickle.loads(pickled_nebs) # raises RecursionError