Add GFZ data services content provider

[nuest]

https://dataservices.gfz-potsdam.de/

I don't think there is an API, so we probably have to parse the landing pages for download links.

http://doi.org/10.5880/GFZ.1.4.2020.003 resolves to https://dataservices.gfz-potsdam.de/panmetaworks/showshort.php?id=escidoc:5183889 which has the following content:

<div class="datafilescolor datasetcontainer">
    <h2><img src="images/external_Ressources_31.png" align="right">Files</h2>
    <span class="citation"><ul class="datasetfiles" style="list-style-type:none;padding:0;margin:0"><li><a class="piwik_download" href="ftp://datapub.gfz-potsdam.de/download/10.5880.GFZ.1.4.2020.003jnvre/">Download data and scripts</a></li><li><a class="piwik_download" href="ftp://datapub.gfz-potsdam.de/download/10.5880.GFZ.1.4.2020.003jnvre/2020-003_Coesfeld-et-al_data-description_VIIRS_DNB_JC.pdf">Data description</a></li></ul></span><span class="review"><ul style="list-style-type:none;padding:0;margin:0"><li><a href="ftp://datapub.gfz-potsdam.de/download/10.5880.GFZ.1.4.2020.003jnvre/">Download data and scripts</a></li><li><a href="ftp://datapub.gfz-potsdam.de/download/10.5880.GFZ.1.4.2020.003jnvre/2020-003_Coesfeld-et-al_data-description_VIIRS_DNB_JC.pdf">Data description</a></li></ul></span>
    </script>
    <p>License:
        <a href="http://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a></p>
</div>

The link leads the user to a FTP page at ftp://datapub.gfz-potsdam.de/download/10.5880.GFZ.1.4.2020.003jnvre/ where there is a ZIP file with the data.

_Pretty complicated 😄 _

https://dataservices.gfz-potsdam.de/panmetaworks/showshort.php?id=escidoc:5148893 is an example with GeoTIFF files

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The page also shows bounding boxes on the search results page, so maybe it's possible to parse that from the HTML page, too.

https://dataservices.gfz-potsdam.de/panmetaworks/showshort.php?id=escidoc:5183889