Create a MeshIO instance according to the kind of `filename`.

    Parameters
    ----------
    filename : str, function or MeshIO subclass instance
        The name of the mesh file. It can be also a user-supplied function
        accepting two arguments: `mesh`, `mode`, where `mesh` is a Mesh
        instance and `mode` is one of 'read','write', or a MeshIO subclass
        instance.
    prefix_dir : str
        The directory name to prepend to `filename`.

    Returns
    -------
    io : MeshIO subclass instance
        The MeshIO subclass instance corresponding to the kind of `filename`.

def any_from_filename(filename, prefix_dir=None):
    """
    Create a MeshIO instance according to the kind of `filename`.

    Parameters
    ----------
    filename : str, function or MeshIO subclass instance
        The name of the mesh file. It can be also a user-supplied function
        accepting two arguments: `mesh`, `mode`, where `mesh` is a Mesh
        instance and `mode` is one of 'read','write', or a MeshIO subclass
        instance.
    prefix_dir : str
        The directory name to prepend to `filename`.

    Returns
    -------
    io : MeshIO subclass instance
        The MeshIO subclass instance corresponding to the kind of `filename`.
    """
    if not isinstance(filename, basestr):
        if isinstance(filename, MeshIO):
            return filename

        else:
            return UserMeshIO(filename)

    ext = op.splitext(filename)[1].lower()
    try:
        format = supported_formats[ext]
    except KeyError:
        raise ValueError('unsupported mesh file suffix! (%s)' % ext)

    if isinstance(format, tuple):
        format = guess_format(filename, ext, format, io_table)

    if prefix_dir is not None:
        filename = op.normpath(op.join(prefix_dir, filename))

    return io_table[format](filename)