Simplified pre-imports in the iPython console

The previous version added too many items to the default shell
dictionary.
Added a new plot dialog

Designed to allow arbitrary NXfields to be plotted against each other.
Still needs debugging.
Added a New File entry to the File Menu

Start of project to allow NeXus files to be constructed through the
GUI. Also changed to throw an error if attempting to rename an item
already in a file, since the NeXus API doesn't allow that.
Fixed problem with adding NeXus groups to the tree

Previously, items wouldn't be added if they were an NXlink item whose
target had already been added. Also changed the right-click methods to
use the mainwindow methods.
Fixing file updates

Fixing issues where the path to a NeXus object didn't match what was in
the file.