Methods for removing attributes from an element

[jennuine]

Signed-off-by: Jenn Nguyen [email protected]

🎉 New feature

Summary

Added methods:

• sdf::Element::RemoveAttribute(string key): Removes a specified attribute from the Element
• sdf::Element::RemoveAllAttributes(): Removes all attributes from the Element

This should be forward ported to sdf10 and sdf11.

Test it

./build/sdformat9/src/UNIT_Element_TEST --gtest_filter=Element.RemoveAttribute
./build/sdformat9/src/UNIT_Element_TEST --gtest_filter=Element.RemoveAllAttributes

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Code check passed (In source directory, run sh tools/code_check.sh)
• All tests passed (See test coverage)
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