Editing nurdle, obsoleting manufactured plastic, and plastic production compound, and adding further relevant classes to the envoPlastics subset

[raissameyer]

As part of our work for UN environment on SDG 14 ...

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...I'm editing the following class such that it follow the genus-differentia structure:

Term: nurdle (xRef #289)

Former parent class: piece of plastic Former definition: A cylindrical shaped pre-production plastic pellet used in manufacturing and packaging.

New parent class: plastic pellet New definition: A plastic pellet which is produced such that its size allows it to be rapidly melted and used as input for further downstream manufacturing processes.

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... I'll also obsolete the following terms:

Term: plastic production compound Former definition: A compound utilized for the production of plastics or polymers. Former comment: Plastics are produced by the conversion of natural products or by the synthesis of primary chemicals generally coming from oil, natural gas, or coal. Plasctic production compounds typically include elements such as carbon, hydrogen, oxygen, nitrogen, chlorine, and sulfur. Reason for obsoletion: This term is ambigious, as it could refer to the compound the plastic is (supposed to be) made of, additives, contaminants, etc. Please instead directly refer to the compound of interest.

Term: manufactured plastic Former definition: A (portion of) plastic is an (portion of) anthropogenic environmental material including any of numerous organic synthetic or processed materials which are primarily composed of thermoplastic or thermosetting polymers of high molecular weight. Reason for obsoletion: This term has been replaced by synthetic plastic as part of a more coherent set of classes as part of the SDG14.1.1 project (see https://github.com/EnvironmentOntology/envo/projects/17)

Please let me know if there is a need for these terms to stay.

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... and adding nurdle, and polyester polymer to the envoPlastics subset.

[pbuttigieg]

Thanks @raissameyer review in the PR