Make the node repository API backend agnostic

[sphuber]

Fixes #2504

Initially, the only backend supported for the file repository of nodes
was the local file system. In the near future, additional backends such
as an object store or a remote file system will be implemented. For this
to work the API of the node repository will have to be adapted because
now it is geared to work with a local file system. To prevent too many
changes after the release of the beta, we already define the API here
that should be able to support the different repository backends that
will be implemented in the future.

[coveralls]

Coverage increased (+0.2%) to 69.715% when pulling e3e4579 on sphuber:fix_2504_node_repository_api_change into 7b1c90a on aiidateam:provenance_redesign.

[giovannipizzi]

I like it!
Only additional comment, for the RemoteData class:

• maybe now the 'disabling' of add_path doesn't work anymore? https://github.com/aiidateam/aiida_core/blob/b22f55155e7a602ac42b00edf62e57e4a458f55b/aiida/orm/nodes/data/remote.py#L45
• does it make sense (maybe) to harmonise then also the API of RemoteData to list, get files from the remote dir, with the same name and type of returned objects (where possible)? Maybe namespacing all that functionality in a remotedata.remotefolder.xxx? Or not really, since in this case we refer to a remote folder on disk (on the supercomputer) while the repository concept is a bit more abstract?

[sphuber]

* maybe now the 'disabling' of add_path doesn't work anymore?

actually, the add_path method no longer exists on the Node class, so there is no point in disabling it. I think we should just remove it. Currently, there exist no Node methods that operate directly on the repository, they are only exposed on the Repository class itself through the repository property. Probably we will need to expose the methods that should be available for the repository, directly on the node itself, which internally will just pipe through, because in the future changes to the repository will anyway need changes on the node instance in the database, so these have to be coordinated by the Node class and cannot be done directly on the repository. That will also make it possible again to disable certain actions for certain node subclasses.

Maybe we should already do this in this PR. Make it impossible to access the repository directly from the node class (i.e. remove the property) and only expose methods that should be callable directly on the node.

[giovannipizzi]

Thanks a lot, that's really a nice step toward the repo improvement!