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>
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