Support SRTM v3.0 tiles

[mibe]

See this issue on the OSM-Wiki talk page.

[hgy59]

Is this still an issue?
I am working with SRTM-3 files since years (Srtm2Osm v13, 2017)

but SRTM-1 support is a desirable feature

[mibe]

I just checked the patch provided in the zip and it seems to me that this is just a matter of different file naming. So it's basically #15 again.