Warn by default on unrecognized elements

[scpeters]

🎉 New feature

Addresses the concern motivating #1035

Summary

It is common practice to add elements to a released version of the SDFormat spec without any changes to the spec version number. This results in different versions of the SDFormat specification being embedded in different versions of libsdformat. For example, //material/shininess was added to SDFormat 1.7 in libsdformat 9.8.0 and to 1.7-1.9 in libsdformat 12.5.0, but the tag is not recognized by libsdformat 12.4.0. I think this can lead to significant user confusion, especially if model and world files begin to proliferate that use elements that aren't recognized by a version of libsdformat shipped with Ubuntu that does not receive feature updates.

I have been brainstorming how to address this concern by adding patch numbers to the SDFormat specification in #1035, but it's a bit complicated. I think an easier intermediate step would be to generate warnings by default when unrecognized elements are found by the parser, which is what this pull request does.

Test it

$ gz sdf --check test/sdf/unrecognized_elements.sdf
Warning [Utils.cc:129] [/sdf/model[@name="joint_incorrect_tags"][@some_attribute="staheousth"]:/Users/scpeters/ws/sdformat/src/sdformat/test/sdf/unrecognized_elements.sdf:L3]: XML Attribute[some_attribute] in element[model] not defined in SDF.
Warning [Utils.cc:129] [/sdf/model[@name="joint_incorrect_tags"]/link[@name="link"]/not_a_link_element:/Users/scpeters/ws/sdformat/src/sdformat/test/sdf/unrecognized_elements.sdf:L6]: XML Element[not_a_link_element], child of element[link], not defined in SDF. Copying[not_a_link_element] as children of [link].
Warning [Utils.cc:129] [/sdf/model[@name="joint_incorrect_tags"]/not_a_model_element:/Users/scpeters/ws/sdformat/src/sdformat/test/sdf/unrecognized_elements.sdf:L8]: XML Element[not_a_model_element], child of element[model], not defined in SDF. Copying[not_a_model_element] as children of [model].
Warning [Utils.cc:129] [/sdf/not_an_sdf_element:/Users/scpeters/ws/sdformat/src/sdformat/test/sdf/unrecognized_elements.sdf:L10]: XML Element[not_an_sdf_element], child of element[sdf], not defined in SDF. Copying[not_an_sdf_element] as children of [sdf].
Valid.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[codecov]

Codecov Report

Merging #1096 (648be0e) into main (f262777) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1096   +/-   ##
=======================================
  Coverage   83.16%   83.16%           
=======================================
  Files         154      154           
  Lines       18846    18846           
=======================================
  Hits        15673    15673           
  Misses       3173     3173           

Impacted Files	Coverage Δ
src/ParserConfig.cc	100.00% <ø> (ø)

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[chapulina]

LGTM. Downstream applications can always revert back to LOG level to suppress these if they want to.