Tie downloaded files to target IDs

[orgua]

Currently the sheep / observer is naming the files by its own ID, but that is no relevant info for the user. they could better use:

• the target ID, or
• the custom ID

i.e. target_01.h5 instead of sheep01.h5.

This untangling could be done in the download-endpoint, no need to rename the files on the actual server.

open question: how to name files when custom IDs are specified?

• target_custom_01.h5
• target_c01.h5
• same name as non-custom? no! as it is confusing
• is there a better way?