Check that inputs match with outputs in `transfer_datasets` API

transfer_datasets(const server& srv, const std::vector<dataset>& sources,

                  const std::vector<dataset>& targets) {

    if(sources.size() != targets.size()) {

        throw std::runtime_error(

                "The number of input datasets does not match the number of "

                "output datasets");

    }

    network::client rpc_client{srv.protocol()};

    const auto rpc = network::rpc_info::create(RPC_NAME(), srv.address());