async preProcess(importData) {
        debug('preProcess');
        for (const importer of this.importers) {
            importData = importer.preProcess(importData);
        }

        return Promise.resolve(importData);
    }
Import Step 2:
Pass the prepared importData through the preProcess function of the various importers, so that the importers can
make any adjustments to the data based on relationships between it
@param {ImportData} importData
@returns {Promise<ImportData>}