Add core schools column without deleting rows

[jacobthill]

We need the core_schools field to plot a distribution of open access publication categories by school, but currently we are deleting any publication that we can't add a core_school to. We should keep these publications and drop them later at the analysis stage, only for visualizations that require school data. An alternative approach could be to add another category label e.g. "no school" that would then get plotted alongside the other school categories.