Update the main `conda` YAML file, i.e. `snakemake` and other potentially relevant packages. This should be the **default** `conda` environment to be used by **all** developers and to be recommended to the user as it will be used for testing as well. Suggested version: `6.7.0`, used together with `mamba`. * [x] update `requirements.yaml` * [x] move `requirements.yaml` out of `config/` * [x] mention in the documentation (i.e. installation instructions)