Store data downloaded from external servers

[bdestombe]

Reduce the depency on uptime external servers, by optionally storing the downloaded files. We experienced several times that deadlines weren't met due to a server being down (e.g., HHNK, KNMI, and REGIS). It would be great if the downloaded files could be stored in a data folder and when running the nlmod functions those files may be used opposed to downloading them from the server.

[dbrakenhoff]

Definitely a good idea. This is already possible for a lot of datasets by rearranging scripts and using the read functions to download data into a data folder first. I do think we could use some added documentation to show users how to write their scripts this way.