Added aspect ratio button to toolbar

This will set the aspect ratio to be equal for x and y values
Various changes to updating HDF5 files

Added update and rename functions to NXFile so that the private
functions within NXFile are no longer referenced outside the class.
However, _readvalue was renamed to readvalue so that it can be
referenced. Also added the equivalent writevalue function. These
changes help to encapsulate IO within the NXFile class to make it
easier to subclass in NXFileRemote.