Create skinny topsoil layer for calculation of temperature and matric potentail at depth of microbial activity

**Is your feature request related to a problem? Please describe.**
The depth of microbial activity does not necessarily align with the depth of the soil layers in the shared `LayerStructure`, see #348 . The `soil` and `litter` model therefore need to get a weighted average between soil layers, see #532 . If the microbial activity falls above the depth of the topsoil layer, there needs to be an interpolation between the surface and the depth of the first deeper layer.

The litter model currently implements a version of that which puts a constant value as the upper boundary condition; this issue asks for the roll out to the whole model. This links to updating the infiltration process #494 but could be done in two steps.

**Describe the solution you'd like**
Add a skinny soil layer, maybe 2-5 cm below ground, to default layer structure and update all models. For the infiltration process, the intermediate solution is to take the depth of the first two layers.

**Describe alternatives you've considered**
We also considered an above ground layer as an upper boundary (as this is also used in the litter model), but this became a bit tricky with the matric potential. However, it would be interesting in the future to think about litter moisture content or relative humidity in litter layer when it comes to flammability and simulations or fires.

Start by reviewing the shared LayerStructure and the soil, litter, and infiltration model work described in issues #348, #532, and #494. Determine how the proposed 2–5 cm skinny layer affects weighted averages, surface interpolation, and the infiltration intermediate solution. Done means the default layer structure and all relevant models consistently handle microbial activity above the first soil layer.

Written by the indexing model from the issue text.