BibField: virtual field aliases amended

* Default alias to calculated fields added to allow calls without
  the '_' prefix. (closes #1530)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>
BibField: new calculated fields

* Adds new calculated fields `_cited_by_count`, `_number_of_comments`,
  and `_number_of_reviews`.

* If you need value from recjson, that might not be there, please
  use `self.get('key')` instead of `self['key']`
  (see  `atlantis.cfg`:`_cited_by_count`)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Co-authored-by: Esteban J. G. Gabancho <esteban.gabancho@cern.ch>
Tested-by: Tibor Simko <tibor.simko@cern.ch>