Added new plotview functions

Added vlines and hlines for multiple x and y values, respectively. Also
added explicit options for setting the end points if it’s not required
to span the whole graph, e.g., hline(y, xmin, xmax).
Updated version number to 0.4.5.

To be ready for another release
Added extra error trapping

Some right-click items on the tree silently fail if the node is not
valid. Now, they produce an explicit error message.
Added logging on initial startup

If NeXpy is invoked from the command-line with a NeXus file as first
argument, the opening will be logged and the root node will be selected
in the tree.