Add at() to the basic_node APIs

[fktn-k]

The subscription operator of the basic_node class, as a convention in STL, doesn't check its own size or a key existence if an object is a container node.
This may lead to an invalid access exception or unwanted creation of a default key-value pair, which is warned in the docs already though.
Such an issue can be avoided by introducing at() API.

[fktn-k]

addressed in the PR #299.