Add more regression tests for upscaling applications

This is an extension to commit d93ec01412. More test models are added and used to test `upscale_perm`. Later, we may include tests also for two-phase upscaling routines based on the framework provided.

Documentation is improved and we use absolute tolerance instead of relative tolerance. The tolerance is also increased to 1e-2 to avoid false negatives.

The PR is tested on RH5 and RH6.