Support srtm files from usgs/EarthExplorer

Currently the srtm provided to S1Tiling must be provided according a shp file. The default one is only for srtm in hgt format.

Personally when I want to retrieve a srtm data, I go to EarthExplorer and I retrieve the tif files. The name is also quite similar: for example n41_w096_1arc_v3.tif. For dt2 format it is the same: n41_w096_1arc_v3.dt2.

The grid is the same only the naming convention is different.

It could be nice to support this file without a new shp file. If I know what I want to process, I will provide the right elevation data.