Add ECIR 1979-1996

[potthast]

ECIR conference misses many old proceedings volumes in the entry: 1979-196.
See: https://en.wikipedia.org/wiki/European_Conference_on_Information_Retrieval

Did some more digging:

ECIR 1997:

• Proceedings download: https://www.scienceopen.com/hosted-document?doi=10.14236/ewic/IR1997.0

ECIR 1998:

• https://www.scienceopen.com/hosted-document?doi=10.14236/ewic/IRSG1998.0

ECIR 1999:

• https://www.scienceopen.com/hosted-document?doi=10.14236/ewic/IRSG1999.0

ECIR 2020:

• https://journals.sagepub.com/doi/epdf/10.1177/016555150002600411
• https://web.archive.org/web/19981203051057/http://lorca.compapp.dcu.ie/BCS-IRSG-97/
  (lists also audio files, but likely lost)