aps_package: various fixes

* updates the script to not add the FFT tag.

* adds a new case for reference harvesting with no
  'mixed-citation' tag.

* adds a new case for proc-type references.

* adds a new case for thesis-type references.

* adds a new case for report-type references.

* fixes _get_doi function.

* handles multiple references with the same label

Signed-off by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>