Issue with large projects separated into subfolders

[Bilokin]

Hello,

thanks for adding the support for multiple sub-directories in the grid projects, i.e. that have multiple sub* folders in their output, but unfortunately, I think there is a bug linked to it: When b2luigi moves the downloaded output from temporary folders of sub00, sub01, subXX, to the output folder, let's call it result/, the contents of sub00 folder is not in the result/, but I suspect that it is in the result/sub00 folder. I tried to run the this code in the python interpreter using sub00 and sub01 input folders and not existing output folder ./result and I get the following tree:

./result
    sub01_content.root
    sub00/
         sub00_content.root

This is not expected by the tasks downstream and I would prefer that the output folder would contain all files from all sub* folders directly. There shouldn't be any conflicts in the file names, but one has to check for that anyway.

[meliache]

Tasks copied over from the duplicate issue #148:

• [x] fix moving so that all root files are flat in the output dir
• [x] encapsulate moving code into its own function (for easier testing)
• [ ] write test with dummy .root files

[meliache]

I now created a draft PR #150 where I changed the moving function to move ROOT files individually into the final output directory, with some more checks in place so it should work, but I still haven't tested it yet, which I will do now.