I now think we should *restrict* the datatype passed
to the filter functions to be *the same* as the one passed to the read
or write function. Everything works with this restriction and it is
easy to define. The "position" parameter functions much like
explicit offsets in files. It specifies the location in memory using
"datatype" to form the (tiled) view in memory.
However, dtype_file_extent_fn can be passed either the etype or any of
the basic datatypes used to construct etype (MPI_LB and MPI_UB not
needed because their size is always 0).
Rajeev