save () - saving collections

[Peyman-N]

Saving a collection as directory should probably consider sub-directories according to schema types (instead of a purely flat list of JSONLDs)

[PatrikBey]

Hey, it would be very helpful if indeed it could retain the same depth of directories as the prior version, i.e. retaining subfolders for each schema type containing the corresponding instances, if the flag for individual files is set to True.

[apdavison]

implemented in #80