Implement new STRM15 version

[remkos]

1. We maintain the topo_srtm15plus variable in rads.xml and rads_add_common.
2. Remove topo_srtm30plus from rads_add_common, but keep it in rads.xml until it has been removed everywhere.
3. Same for topo_dtu10
4. We only update the AUX files (Download and install all the new AUX files #199) so that the content will from that time on be with the new version.
5. This then gets updated with a full update of all new variables in the RADS data base (Add new GDR-G variables to all data sets #204 ).

[remkos]

1. Done
2. Implemented by a137d06
3. Implemented by 11edd24

[remkos]

4. Implemented by Download and install all the new AUX files #199
5. Being implemented by Add new GDR-G variables to all data sets #204