Testing if slab works for mpi4py-fft

test_slab.py

from numpy import *
from mpi4py import MPI
from mpi4py_fft.mpifft import PFFT, Function


N = array([25, 28, 23], dtype=int)

for slab in range(3):
    fft = PFFT(MPI.COMM_WORLD, N, axes=(0,1,2), collapse=False, slab=slab)
    u = Function(fft, False)
    u[:] = random.random(u.shape).astype(u.dtype)
    u_hat = Function(fft)
    u_hat = fft.forward(u, u_hat)
    uj = zeros_like(u)
    uj = fft.backward(u_hat, uj)
    assert allclose(uj, u)
    fft.destroy()
    MPI.COMM_WORLD.barrier()
    if MPI.COMM_WORLD.Get_rank() == 0:
        print('Slab ', slab, ' is ok')