Download of unstructured data via backend module and THREDDS

Within this user story we want to define a work flow how to generate a download URL via the backend module.

The outcome of this user story is a work flow for such a generation of temporary URLs that also takes the permission system into account. One potential idea is to

1. generate an automatically updating entry on the THREDDS-catalog (datasetScan).
2. The backend module extracts the data from the unstructured grid,
3. uploads the extracted data via SFTP,
4. and sends the URL for HTTP download via THREDDS
5. Alternativley, let the user choose a (structured) WCS target and serve the unstructrured data as in #31

As usual, the development team will define the work flow in more details, presents it to the team, revises it and presents it again.

Note: Select data for unstructured grids via xoak

related User Story (Hereon internal access only): #216