More functionality from exporters could move to tvb-storage

Ideally, an exporter should just call: StorageInterface.export_some_data(data, project) and tvb_storage should take care of gathering the data and prepare it for export in the data_export_folder (see ExportManager), returning directly a file path towards the H5 or the ZIP file that should be downloaded.

The code needs a small refactor in order to achieve this. Currently, within exporters, we have multiple calls towards the StorageInterface to build the export folder, copy files, write zip files, and cleanup after export. These could all move inside the tvb_storage.