Added sectionIdentifier(for:) method for data sources#34

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absoftware wants to merge 3 commits intora1028:masterfrom absoftware:sections

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Commits on Dec 7, 2020

Added method sectionIdentifier(for section: Int) -> SectionIdentifierType? for data sources.
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Added method sectionIdentifier(for section: Int) -> SectionIdentifierType? for Cocoa data sources.
absoftware
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Added tests for section identifiers from data sources.
absoftware
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