Reader enhancements

[xemasiv]

Add Reader.asMappedQuery()

          let mapped_megacenters = {};
          megacenters.entities.map((_entity, _index) => {
            let megacenter_key = megacenters.keys[_index].name || megacenters.keys[_index].id;
            mapped_megacenters[megacenter_key] = {
              ..._entity
          });