Add statistics to abiotic variables

As discussed in #425, we need the `abiotic`, `abiotic_simple`, and `hydrology` model to return statistics of microclimate variables to represent the variability over a monthly time step.

We decided to introduce an additional axis `climate_stats` to add mean, min, and max to the requested variables (air temperature, soil temperature, vpd, soil mositure).

We considered alternatively to add variables for each statistic, but that would blow up the code unnecessarily. With the setup of numpy arrays, we can hopefully run all functions smoothly for the additional dimension without extra looping or similar