Bug fixes

-   Fix the swarm of warning messages that appear when downscaling
    images of integer type by the averaging method.

Other improvements

-   The default downscaling method is now chosen automatically based on
    the type of dataset: image uses local averaging, segmentation
    uses striding.
-   The command-line interface of mesh-to-precomputed was changed to
    work with Accessors, instead of being restricted to files. The
    command also now reads and writes the info file to make sure that
    Neuroglancer knows where to find the mesh fragments.
-   Introduced a new command link-mesh-fragments to create the small
    JSON files that are required to link segmentation labels to
    mesh fragments.