REACH-29 Save workspace into client file system

In the Flood we can save home workspace and custom node workspaces as files. And later we can restore this workspaces from files.
Save: the Dynamo receives a message about saving, saves all workspaces as files and sends them back to the Flood.
Upload: the Dyname receives a file as byte array, saves them as a file, upload this file, sends all needed for restoring the workspace data to the Flood
This feels a little not right. Under what scenario would we find ```NewCustomNodeWorkspace``` being called when the ```id``` is already having a corresponding ```CustomNodeWorkspaceModel```? Even if that is the case, should we not reuse the existing ```CustomNodeWorkspaceModel``` instead of making a new one?

This needs extensive documentation (you can put ```///``` comments above ```NewCustomNodeWorkspace``` method), it is unclear right now to me.