New function as alternative to `operator[]` for read and write

[Spixmaster]

Example:

YAML::Node node = YAML::LoadFile(constant::file::configuration);
const double commission_factor = node["factor"].as<double>();
node["factor"] = 4.532;

It would be beneficial to have an alternative function instead of the overloaded operator operator[] to please the clang-tidy recommendation to not use overloaded operator[], https://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines/pro-bounds-avoid-unchecked-container-access.html. This is needed for read and write operations.

This makes sense as in the STL operator[] does not throw exceptions but rather aborts the programme and thus is not recommended.

[KitsuneRal]

This clang-tidy rule doesn't apply to associative classes of STL by default, for a good reason; and YAML::Node is effectively an associative container. I'd rather add YAML::Node to cppcoreguidelines-pro-bounds-avoid-unchecked-container-access.ExcludeClasses in .clang-tidy.

[SGSSGene]

There is a related Issue #980 which seems to have the same issue.