Add file size information to entries for files in the KG

Migrated from GitHub issue HumanBrainProject/hbp-validation-framework#278, opened by @shailesh on 2020-07-27.

GET/HEAD end-points cannot be accessed for CSCS storage. So it would be nice to have info such as file sizes stored on their KG entries.

@apdavison intends to write a script to add the file size information to the KG