Memory leak output increments

Created originally on Bitbucket by paugier (Pierre Augier)

I get a nice memory leak when using output.increments with MPI.

This issue could be related to

https://bitbucket.org/fluiddyn/fluidsim/issues/27/memory-leak-with-mpi-h5py-state_phys

but here nothing to do with parallel h5py (that I don't use here).

The memory leak is due to comm.allreduce used with np.array in operators2d.py.

See https://bitbucket.org/mpi4py/mpi4py/issues/119/memory-leak-allreduce

A workaround is to use Allreduce instead of allreduce.