numbers starting at
index <code>local_i_start</code>; e.g. if only a single process is used,
then <code>local_ni</code> will be <code>n0</code> and <code>local_i_start</code> will
be <code>0</code>.  Similarly for the output, with <code>local_no</code> numbers
starting at index <code>local_o_start</code>.  The return value of
<code>fftw_mpi_local_size_1d</code> will be the total number of elements to
allocate on the current process (which might be slightly larger than
the local size due to intermediate step