Clean up custom base class

[barcode]

(Second part of the points listed in #3110 (comment), the rest is done in a different PR to prevent mixing of separate issues / features)

• Warn users about potential conflicts with newer versions, i.e., base members should avoid generic names to avoid clashes with future member additions to basic_json.
• (Run reuse.)

Also adds a new method as_base_class to make it easier to access members of a custom base class shadowed by nlohmann::basic_json

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 100.0%. remained the same when pulling 0f6d471 on barcode:clean_up_custom_base_class into 5d27543 on nlohmann:develop.

[barcode]

Work on this PR is done and it only needs a review.