Make folder download less dangerous.

[axlbonnet]

Currently the download folder operation take a local directory as parameter to store the zipped folder.

But this in not the directory in which the zip will be put as could be expected, but a local temp directory to create the zip content, that will be eventually deleted.
So clients MUST NOT give a existing folder for this result parameter because grida will delete it.

Either :

• modify this method behavior (and all the places where it is used, with special attention in existing workflows)
• deprecate the current method and create a new one