No ippl class

Remove the class Ippl and replace with global initialize and finalize functions. This way we get rid of the static variables.

Old initialisaton:

int main(int argc, char* argv[]) {
    Ippl ippl(argc, argv);

    // some code
}

New initialisation:

int main(int argc, char* argv[]) {
    ippl::initialize(argc, argv);
    {
        // some code
    }
    ippl::finalize();
}