Update srtm.py to get hole-filled product

[scottyhq]

I noticed the srtm.py fetches data from "http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/". There are then ways to fill the holes and shade (#396).

Since 11/2013, A newer version of void-filled SRTM was made available via NASA/USGS LPDAAC.

The server url is new, but the syntax is similar to before (without region subfolders):

"http://e4ftl01.cr.usgs.gov/SRTM/$type.003/2000.02.11/$file" Where $type is either 'SRTMUS1' for 30m data over the US, or 'SRTMGL3' for 90m data over the rest of the world. And $file is essentially the same naming scheme (e.g. N18E145.SRTMGL3.hgt.zip). Also, there is other imagery available via the same server, which might be of interest to users.

I could take a stab at a pull request for getting the new data, but am pretty new to pull requests...

[ajdawson]

Is the SRTMGL3 data product equivalent to the product currently in use (i.e. same resolution etc. just with gaps filled)? If so this could be a straight swap.

I've constructed a JSON lookup file (using this parser) for this data set and verified it can be slotted into cartopy with minimal changes. A few things to think about:

• deprecating the fill_gaps() function
• caching: SRTM tiles are cached, so users may see old data products if they used the previous data set, perhaps just changing a name would get around this

[QuLogic]

This will be fixed by #690.