WIP: Regroup download

[kimicol]

Regroup all downloads of one instrument in one zip file

[ofaurax]

Several tasks here:

• [ ] define what should be downloaded: a program? which one?
• [ ] define the instrument for which we need the download: "clarinette 1" is clear, but what about "Oboe 3" when there's only one part?
• [ ] for each score, identify which file is relevant. We will have work to standardize names across existing scores and future ones.
• [ ] check if we don't have memory and file size limit when creating the download file (PHP has a limit in file size, and also on script execution time)