getGroups() {
    const groups = _.get(this.component, 'rowGroups', []);
    const sizes = _.map(groups, 'numberOfRows').slice(0, -1);
    const indexes = sizes.reduce((groupIndexes, size) => {
      const last = groupIndexes[groupIndexes.length - 1];
      return groupIndexes.concat(last + size);
    }, [0]);

    return groups.reduce(
      (gidxs, group, idx) => {
        return {
          ...gidxs,
          [indexes[idx]]: group
        };
      },
      {}
    );
  }
Create groups object.
Each key in object represents index of first row in group.
@returns {object} - The groups object.