multiprocessing hangs in iterative solver

[bonfus]

For some reason, the second time (?!) the code hits this line:

https://github.com/stefanch/sGDML/blob/master/sgdml/utils/desc.py#L345

during sgdml all, multiprocessing imap blocks the execution and ctrl+c gives this backtrace:

^C[CRIT] Traceback (most recent call last):
         File "/usr/lib/python3.9/multiprocessing/pool.py", line 853, in next
           item = self._items.popleft()
       IndexError: pop from an empty deque

       During handling of the above exception, another exception occurred:

       Traceback (most recent call last):
         File "/home/pie/Venvs/sgdmlenv/lib/python3.9/site-packages/sgdml/cli.py", line 1083, in train
           model = gdml_train.train(
         File "/home/pie/Venvs/sgdmlenv/lib/python3.9/site-packages/sgdml/train.py", line 927, in train
           R_desc, R_d_desc = desc.from_R(
         File "/home/pie/Venvs/sgdmlenv/lib/python3.9/site-packages/sgdml/utils/desc.py", line 345, in from_R
           map_func(partial(_from_r, lat_and_inv=lat_and_inv), R)
         File "/usr/lib/python3.9/multiprocessing/pool.py", line 858, in next
           self._cond.wait(timeout)
         File "/usr/lib/python3.9/threading.py", line 312, in wait
           waiter.acquire()
       KeyboardInterrupt

Disabling parallelism in this function is an acceptable workaround (not much time spent here).

Using numpy compiled with MKL support and 12 cores.