Added a treeview.node property

This means that the currently selected node can be accessed from the
console by ‘treeview.node’
Fixed bug in add_data

Even though a node was selected, the dialog was assuming it wasn’t.
Added NXPlotView to the console imports

This allows a new plot view to be created from the terminal without
additional imports.
Improved FitDialog layout

I’ve removed the separate plot window, which caused resize problems -
fits are now plotted in a regular plot window. I’ve also adjusted the
scroll area so that it only operates on the parameter grid, which makes
the layout less confusing.
Added extra function methods

These simplify the use of the Fit class in scripts.