Minor modelbuilder improvements

[veenstrajelmer]

Todo:

• set overwrite=False again. Add note to exercises that overwrite should be true when changing the domain
• add note to modelbuilder notebook: "you need to create a cmems and cds account at ..." (see below)

Add explanatory text:

The model requires some basic input data. We will make use of publicly available data from Copernicus Programme of the European Union. To access this data you need to make two accounts for the [Copernicus Marine Service](https://marine.copernicus.eu/) and the [Climate Data Store](https://cds.climate.copernicus.eu/#!/home). Follow the instructions below to set up your accounts.

Climate Data Store
To download the meteorological reanalysis data from ERA5 we will use the CDS API. Follow the steps to register and generate a personal key in the block "Install the CDS API key". dfm_tools will request your key ({uid}:{api-key}) and save it in your home directory ("[userfolder]/.cdsapirc"). This key will be read by the API when you start a download request. 

Copernicus Marine Service
Register with the Copernicus Marine Service [on this page](https://data.marine.copernicus.eu/register).
dfm_tools will request you to enter your credentials and they will be saved in your home directory ("[userfolder]/CMEMS_credentials.txt")