Fix linting errors & warnings and fix JSdocs in DataPackage (DataPackage cleanup, part 1)

[robyngit]

Other than fixing linting errors and JSdocs, this PR:

• Removes unused properties from DataPackage collection: sourcePackages, derivationPackages, relatedModels. These are used in PackageModel, and we should check if those properties and the logic that uses them are required in the DataPackage (since we plan deprecate PackageModel).
• Converted some underscore methods (e.g. _.each) to ES6 methods (e.g. .forEach), but there are still many to convert.

This is the first step in issue #2567