Clean up sky map URLs in VOEvents for unauthenticated access

Reduce the number of different URLs that are given in the sky map section to two: one for the FITS file and one for the PNG file. Rename to skymap_fits and skymap_png.

        <Group type="GW_SKYMAP" name="bayestar">
            <Param name="skymap_fits" dataType="string" value="..." ucd="meta.ref.url" unit="">
                <Description>Sky Map FITS</Description>
            </Param>
            <Param name="skymap_png" dataType="string" value="..." ucd="meta.ref.url" unit="">
                <Description>Sky Map image</Description>
            </Param>
        </Group>