Centralise dem support

[sixy6e]

The workflow requires several elevation datasets to retrieve height values. Inherited from the legacy system which used different resolutions for different purposes, some being read a single pixel, and the lower resolution datasets yielded faster results for this purpose.

By changing to the definitive source that is used for terrain illumination correction, simplifies the workflow and removes a parameter/argument from the workflow, as well as simplifying the metadata by tracking a single elevation source.