Update Likelihood xml Interface

[jasercion]

Likelihood's previous references to XmlBase and Xerces-C need to be updated to use the rapidXml Interface

[jasercion]

Likelihood/XmlBuilder.h
Likelihood/XmlParser.h
src/XmlBuilder.cxx
src/XmlParser.cxx

These files encapsulate the xml parser prototype class. They are very simple and have now been updated. The most work will likely have to be done in

src/SourceModel.cxx
src/SourceFactory.cxx

As these files contain the actual routines that were previously calling xmlBase and the Xerces-C api directly. I've begun work on both of these, in particular SourceFactory as that is what does most of the heavy lifting.