GroundwaterFlow: support exponential decline of hydraulic conductivity with depth

[verseve]

Aquifers in GroundwaterFlow have a fixed (average) horizontal hydraulic conductivity k per grid cell. For the application of this concept in (steep) sloping terrain an exponential decline of k with depth would make sense. These areas are generally characterized by relatively thin soils above (strongly) weathered bedrock and such a profile can be captured by an exponential decline of k.