Export/Import of DT with related entities

From a DT overlay in TVB web gui currently we can export a single H5 file.

This functionality should be kept, and another button should be added in the same area "Export With References".

This new export should produce a ZIP of multiple H5 files, including the current DT, plus all linked entities (for example together with a RegionsMapping H5 both a Connectivity and a Cortical Surface H5 should be zipped).

The mirror Import (from H5/ZIP) should be enhanced.