Use pkg_resources to locate tree icons

The old method of using __file__ to identify file paths is now
deprecated.
Remove duplicate tree entries when created in the shell

If a node with the same path as an existing one is created in the
shell, then node._item was recreated without deleting the old one, so
duplicate items appeared on the GUI tree although only one was in the
NeXus tree. The new function deletes duplicate siblings from the GUI.
Use pkg_resources to locate icons

The old method of using __file__ to identify file paths is now
deprecated. The app icon is now set in consoleapp.py rather than
mainwindow. Also fixed a bug in using the GUI to rename a null item.
Added __setitem__ to NXfile class

This is needed to rename an item within an existing HDF5 file.
Moved save_fit functionality to the Fit class

It was defined within the GUI Dialog class, but it is also useful when
the Fit class is used apart from the GUI.