Added some convenience constructors and operators to AMElement.  Started work on making AMSelectablePeriodicTable the main replacement for XRFPeriodicTable.  Created the shell of AMSelectableElement which will replace XRFElement.
Did some code cleanup in AMPeriodicTable and AMPeriodicTableView.  Fleshed out a bunch of AMSelectableElement and fixed a small range bug in AMSelectablePeriodicTableView.