Saving workspace functionality

Saving workspace functionality:
- the Dynamo receives a message about saving, saves all workspaces as files and sends them back to the Flood.

Part of changes that had to be uploaded with "Changing the way of geometry transmission" but wasn't:
- receiving a geometry message, process it and send back the geometry data

Enhancements of the code (there is not a lot of them, so I add them into this PR):
- checking for not null, correct guid, removing old code, etc
There are unnecessary changes in this file, could you go back to the original code and just add this instead?

```cs
NodeModel node = Model.GetModelInternal(nodeId) as NodeModel;
if (node == null)
    return;

// Other existing code...
```