Minor enhancements

1) Added an ‘nxentry’ property to return the parent NXentry group of an
NXobject.
2) Removed error checking for plotting a group with a signal but
without axes defined.
3) Fixed bug in convert_index, which would return an incorrect value
when one of the indices is None.
Major efficiency gains in NXPlotView

1) Before, updating an image often involved repotting it, even when
only changing the color scale. Now, matplotlib update routines, e.g.,
set_clim and set_norm, are utilized to save in replot times. This has
speeded up the movie displays considerably.
2) Made improvements to handling changes to autoscale and changing and
resetting plot limits
3) Now uses either imshow or pcolormesh, depending on whether the plot
pixels are a constant size or not.
4) Changed name of NXPlotView set_limits and reset_limits to
set_plot_limits and reset_plot_limits, to harmonize with new
set_data_limits function.