Add SRTM-derived 1-second DEMs for Australia on Sandbox

[robbibt]

DEMs are a fundamental piece of ancillary data that can be combined with earth observation data to extract better insights into environmental change and processes.

However, we do not currently have a DEM accessible via the DEA Sandbox (despite this being one of the first products added to DigitalEarthAfrica). This forces users to download DEM data themselves, which can be impractical given the volume of data involved (up to 30gb for continental Australia), and makes it more difficult to write reproducible code.

Geoscience Australia publishes a set of 1-arc second (~30m) resolution DEMs that could be suitable for indexing into the Sandbox: https://ecat.ga.gov.au/geonetwork/srv/eng/catalog.search#/metadata/72759

The data is available via a Creative Commons Attribution 4.0 International Licence, so should be able to be made available via the Sandbox.

Note: the data linked above are the 1-second Digital Elevation Models, not the Digital Surface Models which are currently restricted to government use

[robbibt]

The data linked above may be the same source data as this product on the NCI (the DEM, not the DSM): https://explorer.dea.ga.gov.au/srtm_dem1sv1_0