Extended get-datafiles.sh to download high-res data

[fox918]

I changed the ./get-datafiles.sh script to embedded the following changes:

• added different optional flags
• enabled downloading data for srtm2sdf-hd from the SRTMGL1 datasource (Nasa)
• added a "Direct Mode" to directly convert downloaded files (useful when running on a server, download the whole earth while working with the already downloaded data)

Issues:

• was tested in Archlinux, no extended testing in Debian
• maybe add something to specify a Lon/Lat-Range for downloading high-res data