docs launching ewatercycle on mistral

For lack of a better place I will store my findings on how to run ewatercycle on Mistral in this issue with the aim that this will be part of the future docs.

On Mistral we can launch a Jupyter Lab server on a compute node using: https://jupyterhub.dkrz.de/hub/login. However, this server does not load Singularity as a default and it is not possible to load Singularity from the terminal or notebooks.

The solution:
Create a shell script that loads Singularity and call this shell script when launching the kernel, see: https://jupyterhub.gitlab-pages.dkrz.de/jupyterhub-docs/posts/2021/2021-02-16_kernels_advanced.html.

The shell script:
`#!/bin/bash

module purge
module load singularity/3.6.1-gcc-9.1.0
. ~/mamba/bin/activate ewatercycle
python -m ipykernel_launcher -f "$1"
`

Run: chmod +x shell_script.sh

Modify: "~/.local/share/jupyter/kernels/ewatercycle/kernel.json" To:

`{
"argv": [
"/path/to/file/shell_script.sh",
"{connection_file}"
],
"display_name": "ewatercycle",
"language": "python"
}
`