setdefault for c++, the missing Python::dict method

setdefault.hpp

/// setdefault(D, k, v) -> D.at(k), and set D[k]=v if k not in D
template<class T, class K, class V>
V& setdefault(T &map, const K &key, const V &default_value)
{
	if (!map.count(key)) {
		map.insert({key, default_value});
	}
	return map.at(key);
}