Improve files copy from Scratch to output dataspaces

[lpellegr]

When dataspaces are used, the copy of files from input dataspaces (input, output, global or user space) to the Scratch dataspace on compute nodes is performed in parallel. However, when a task has finished, the copy of files from the Scratch space to the output dataspaces (output, global or user space) is performed in parallel per FileSelector which limits the parallelism to the number of files identified per selector.

See TaskProActiveDataSpaces#copyScratchDataToOutput.