#298 Add at() API to the basic_node class

[fktn-k]

This PR addresses #298 by implementing the new basic_node's at() API.
To validate the implementation, test suite has also been updated to cover the newly added lines/branches.
The API documentation has also been updated with the descriptions/examples exclusively for the new API.

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Pull Request Checklist

Read the CONTRIBUTING.md file for detailed information.

• [x] Changes are described in the pull request or in a referenced issue.
• [x] The test suite compiles and runs without any error.
• [x] The code coverage on your branch is 100%.
• [x] The documentation is updated if you added/changed a feature.

Please don't

• The C++11 support varies between different compilers and versions. Please note the list of supported compilers. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual Studio 13.0 and earlier are known not to work due to missing or incomplete C++11 support. Please refrain from proposing changes that work around these compiler's limitations with #ifdefs or other means.
• Please refrain from proposing changes that would break YAML specifications. If you propose a conformant extension of YAML to be supported by the library, please motivate this extension.
• Please do not open pull requests that address multiple issues.

[coveralls]

Pull Request Test Coverage Report for Build 8422880673

Details

• 82 of 82 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 8408009895:	0.0%
Covered Lines:	3018
Relevant Lines:	3018

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💛 - Coveralls