code: function (importData) {
          var ops = [];
          _.each(this.importers, function (importer) {
              ops.push(function () {
                  return importer.preProcess(importData);
              });
          });
  
          return pipeline(ops);
      }
docs: 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)}