Refactor duplicated code in Xpp3Dom and Xpp3DomUtils

[belingueres]

The following methods are duplicated (or nearly identical):

• mergeXpp3Dom
• writeToSerializer
• isEmpty
• isNotEmpty

Move all the code to only one of the classes. Deprecate methods in the other class, delegating its implementation to the first class.

[gnodet]

@belingueres @michael-o The PR deprecates Xpp3DomUtils in favour of Xpp3Dom which contains the needed methods.