Do not display 3D view button if collection is empty. Closes #73
Merge pull request #81 from chrisfilo/fix/3d_empty

Do not display 3D view button if collection is empty. Closes #73
redirect back to the collection if pycortex is called and collection is
empty
Merge pull request #82 from chrisfilo/fix/3d_empty

redirect back to the collection if pycortex is called and collection is empty