Reimplement calculation of reflectance from non-HRIT datasets

[spanezz]

The previous prototype implementation was removed in 42796f4 due to GDAL issue 5989 that caused it not to work from GDAL 3.4 onwards.

Since this has never gone into production, the easy fix for #29 was to remove the prototype implementation, and postpone redesigning this feature until there is a new demand for it. At that point we can reassess our options, based on the status of GDAL and an updated analysis of production needs.

Until then, this issue can act as background information, with a link to the previous implementation and to its related GDAL issue.