#240 implement node comparison

[fktn-k]

To prepare implementation in which non-string nodes are also supported as mapping keys, basic_node class operators for comparison have been added in this PR.
Implementing those operator will enable basic_node class to be used as std::map::key_type.
For detailed information about those operators, visit the API documentation pages.
Furthermore, unit test cases and documentation have also been updated.

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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

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[coveralls]

coverage: 100.0%. remained the same when pulling a3718a6eb0f70a35b84b48f9908caffad62171cf on feature/240_implement_node_comparison into 8065a3fba90ce9feffbd9168c50a204f0921ca04 on develop.