Evaulation metrics to add

#### Basics
- [x] mean bias + RMSE/MAE
- [x] histogram/PDF + PSS
- [x] interannual variability
- [x] multi-year variability
- [x] five-year maximum
- [x] percentiles
- [x] change signal
- [x] wet day frequency
- [x] temporal autocorrelation
- [x] climatology bias
- [x] multiannual drought
- [x] P-T cross-correlation
- [x] Seasonal cycle
- [x] 1-in-10-year event

#### icclim indices (see [here](https://icclim.readthedocs.io/en/stable/references/ecad_functions_api.html#module-icclim._generated_api) and [here](https://www.ecad.eu/documents/atbd.pdf) for options)
- [x] Temperature variability (e.g. WSDI, CSDI)
- [x] Rainfall distribution (e.g. R95pTOT, R99pTOT, R20mm, R10mm)
- [x] Frost days

(Each as a separate notebook in the `results/` directory.)