Error when running `AnalysisFromFunction()` on more processes than frames

[luponzo86]

Expected behaviour

Successfully running AnalysisFromFunction() on all available CPUs by setting n_jobs=-1 even for very small trajectories.

Actual behaviour

A Warning is raised:

/srv/home/lponzoni/anaconda3/envs/ifpe/lib/python3.7/site-packages/pmda/parallel.py:360: UserWarning: run() uses more blocks than frames: decrease n_blocks
  warnings.warn("run() uses more blocks than frames: "
/srv/home/lponzoni/anaconda3/envs/ifpe/lib/python3.7/site-packages/numpy/core/_asarray.py:83: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray
  return array(a, dtype, copy=False, order=order)

but the code runs anyway until an error is thrown:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

<omissis>

/srv/home/lponzoni/anaconda3/envs/ifpe/lib/python3.7/site-packages/pmda/parallel.py in run(self, start, stop, step, n_jobs, n_blocks)
    398                 # save the frame numbers for all blocks
    399                 self._blocks = _blocks
--> 400                 self._conclude()
    401         # put all time information into the timing object
    402         self.timing = Timing(

/srv/home/lponzoni/anaconda3/envs/ifpe/lib/python3.7/site-packages/pmda/custom.py in _conclude(self)
    101 
    102     def _conclude(self):
--> 103         self.results = np.concatenate(self._results)
    104 
    105 

<__array_function__ internals> in concatenate(*args, **kwargs)

ValueError: all the input arrays must have same number of dimensions, but the array at index 0 has 2 dimension(s) and the array at index 10 has 1 dimension(s)

Code to reproduce the behaviour

I could not find MDs to run an example on (I had problems installing MDAnalysisTests, see issue #3084) but it basically happens when AnalysisFromFunction() is run on a trajectory with n frames and n_jobs is set to a value greater than n, or n_jobs = -1

This is not a big deal, but it was hard to debug and I wanted to report it.

Currently version of MDAnalysis: 1.0.0

pmda version: 0.3.0