cache elevation data of world wind java using mapproxy

I finally find a way to cache the elevation data of world wind java with mapproxy.
Old version of mapproxy did not handle tiff 16bit, and mapproxy-1.11 with pillow-5.2 do.

Keypoints:
1. get elevation tiles in image/tiff
2. store the tiles in image/png, mode I
3. deliver the tiles in image/ppm

==mapproxy.yaml==
layers:
  - name: Elev
    title: NASA_SRTM30_900m_Tiled in TIFF
    sources: [Elev_cache]

caches:

  Elev_cache:

    sources: [Elev_wms]

    format: png16

    request_format: image/tiff

    grids: ['Elev_grid']

    image:

      resampling_method: bicubic

    cache:

       type: sqlite

       directory: /opt/mapproxy/cache/elev

sources:

   Elev_wms:

      type: wms

      supported_srs: ['EPSG:4326']

      wms_opts:

        version: 1.3.0

      req:

         url: https://worldwind26.arc.nasa.gov/elev

         layers: NASA_SRTM30_900m_Tiled

grids:

  Elev_grid:

    srs: EPSG:4326

    num_levels: 12

    origin: nw

globals:

  # image/transformation options

  image:

      paletted: False

      formats:

         png16:

            format: image/png

            mode: I

==end mapproxy.yaml==

And EarthElevations2.xml  can be used with the changes:

        <ImageFormat>image/ppm</ImageFormat>

        <DataType type="Int16" byteOrder="LittleEndian"/> 

        <FormatSuffix>.ppm</FormatSuffix>