Create and store data inside projects when opening workspaces

Changes to _context.py:
* Context has been extended to contain a WorkspaceProjectCache, as there
  are times when we want to use it before a Workspaces can be initialised
  (looking up a WorkspaceProject to find the directory that the project is
  in)

Changes to _stream.py:
* Removed staging the elements from workspace_open() and workspace_reset()

Changes in _workspaces.py:
* A new WorkspaceProject contains all the information needed to refer back
  to a project from its workspace (currently this is the project path and
  the element used to create this workspace)
* This is stored within a new WorkspaceProjectCache object, which keeps
  WorkspaceProjects around so they don't need to be loaded from disk
  repeatedly.
* Workspaces has been extended to contain the WorkspaceProjectCache, and
  will use it when opening and closing workspaces.
* Workspaces.create_workspace has been extended to handle the staging of
  the element into the workspace, in addition to creating the equivalent
  WorkspaceProject file.

This is a part of #222