Begun replacing class definitions with inheritance-based classes
taking a brute-force approach in defining conversion methods.
Conversion methods between XML and S4 explicitly assign attributes
and values to S4 slots, rather than using the clever XML package
functions to guess what goes where (since that approach cannot
include a call to callNextMethod, which allows us to recursively
go down the inheritance classes. The up side of this approach is
that you don't have to think much to write methods, it's merely verbose.)