Xaod

Work in progress, issuing a pull request just to hear
comments&suggestions from our xaod experts :-)

The main idea is to:
- start on top of the `ntup-common` branch
- keep the objects created by SusyNtMaker, but to drop
all the D3PDReader-related classes (namely SusyD3PDInterface,
D3PDAna),
- have SusyNtMaker inherit from a new class XaodAnalysis
that will almost maintain the functions/interface of D3PDAna, but
internally it will deal with Xaod and SUSYObjDef_xAOD

Does it sound reasonable?