Sampler: Default name of output JSON file has two extensions

In the constructor of the class Sampler we need to replace

std::string fname = "samples_" + std::to_string(comms_.island_id) + ".json";
std::string resultFile = args->getArg<std::string>("outfile", fname, false);

with

std::string resultFile = args->getArg<std::string>("outfile", "output", false);

Otherwise the default output JSON file is wrong.