Made changes to ADIOS Class to return references and smart pointers

rather than handlers

Group&, Method& and the polymorphic std::shared_ptr<Engine> will be
returned by DeclareGroup, DeclareMethod and Open.

Added Group functions DefineVariable to be a template
Now using adios::Var when creating a variable (still a variable name
string).

Uses more C++ OO + templates features

Compiled and tested with ./examples/hello/helloWriter_OOP.cpp , mpi and
no-mpi versions