modifies(_) {
      const hash = this.fields || (this.fields = {});
      if (isArray(_)) {
        _.forEach(f => hash[f] = true);
      } else {
        hash[_] = true;
      }
      return this;
    }
Marks one or more data field names as modified to assist dependency
tracking and incremental processing by transform operators.
@param {string|Array<string>} _ - The field(s) to mark as modified.
@return {Pulse} - This pulse instance.