Make the 'Home' tab puts x and y to their limits

Previously, if the data were plotted with a new z while focused on a
narrow area in x and y, the ‘Home’ button would reset to this narrow
area rather than the whole range.
Added __getstate__ and __setstate__

These functions have been added to NXobject so that all NeXus objects
can be serialized, e.g., with the Pickle module. This is not
particularly useful at the moment, but it would allow inter-process
communication of NeXus objects in the future.