Add core ranking

* for all 42 bib entries with the keyword schubotzCv

gipp.bib

@@ -890,6 +890,7 @@
   location   = {Santiago, Chile},
   publisher  = {ACM},
   booktitle  = {Proceedings of the 38th International ACM SIGIR Conference on Research and Development in Information Retrieval},
+  core       = {A*},
   date       = {2015},
   doi        = {10.1145/2766462.2767787},
   file       = {:disSigir15.pdf:PDF},
@@ -964,6 +965,7 @@
   location   = {Pisa, Italy},
   publisher  = {ACM},
   booktitle  = {{P}roceedings of the 39th {I}nt. {ACM} {SIGIR} {C}onference on {R}esearch and {D}evelopment in {I}nformation {R}etrieval},
+  core       = {A*},
   date       = {2016},
   doi        = {10.1145/2911451.2911503},
   file       = {:disSigir16.pdf:PDF},
@@ -1031,6 +1033,7 @@
   location  = {Newark, New Jersey, USA},
   publisher = {ACM},
   booktitle = {{P}roceedings of the 16th {ACM}/{IEEE}-{CS} {J}oint {C}onference on {D}igital {L}ibraries ({JCDL})},
+  core      = {A*},
   date      = {2016},
   doi       = {10.1145/2910896.2910908},
   isbn      = {978-1-4503-4229-2},
@@ -1251,6 +1254,7 @@
   publisher = {ACM},
   url       = {http://doi.acm.org/10.1145/3132847.3133144},
   booktitle = {Proceedings of the 2017 {ACM} on Conference on Information and Knowledge Management, {CIKM} 2017, Singapore, November 06 - 10, 2017},
+  core      = {A},
   date      = {2017},
   doi       = {10.1145/3132847.3133144},
   keywords  = {schubotzCV,preprint},
@@ -1325,6 +1329,7 @@
   publisher = {ACM},
   biburl    = {https://dblp.org/rec/bib/conf/recsys/SchwarzerBSMG17},
   booktitle = {Proceedings of the Eleventh {ACM} Conference on Recommender Systems, RecSys 2017, Como, Italy, August 27-31, 2017},
+  core      = {B},
   date      = {2017},
   doi       = {10.1145/3109859.3109981},
   keywords  = {schubotzCV,preprint},
@@ -1374,6 +1379,7 @@
   url       = {http://doi.acm.org/10.1145/3197026.3203899},
   biburl    = {https://dblp.uni-trier.de/rec/bibtex/conf/jcdl/HamborgBSLG18},
   booktitle = {Proceedings of the 18th {ACM/IEEE} on Joint Conference on Digital Libraries, {JCDL} 2018, Fort Worth, TX, USA, June 03-07, 2018},
+  core      = {A*},
   date      = {2018},
   doi       = {10.1145/3197026.3203899},
   keywords  = {SchubotzCV,preprint},
@@ -1473,6 +1479,7 @@
   location  = {Ann Arbor, MI, USA},
   biburl    = {https://dblp.uni-trier.de/rec/bibtex/conf/sigir/MeuschkeSSG18},
   booktitle = {Proceedings of the International ACM SIGIR Conference on Research and Development in Information Retrieval (SIGIR)},
+  core      = {A*},
   date      = {2018},
   doi       = {10.1145/3209978.3210177},
   file      = {Meuschke2018a.pdf:Meuschke2018a.pdf:PDF},
@@ -1578,6 +1585,7 @@
   url       = {http://doi.acm.org/10.1145/3197026.3197058},
   biburl    = {https://dblp.org/rec/bib/conf/jcdl/SchubotzGSMCG18},
   booktitle = {Proceedings of the 18th {ACM/IEEE} on Joint Conference on Digital Libraries, {JCDL} 2018, Fort Worth, TX, USA, June 03-07, 2018},
+  core      = {A*},
   date      = {2018},
   doi       = {10.1145/3197026.3197058},
   keywords  = {SchubotzCV,preprint},
@@ -1617,6 +1625,7 @@
   author    = {Meuschke, Norman and Stange, Vincent and Schubotz, Moritz and Karmer, Michael and Gipp, Bela},
   location  = {Urbana-Champaign, Illinois, USA},
   booktitle = {Proceedings of the ACM/IEEE-CS Joint Conference on Digital Libraries (JCDL)},
+  core      = {A*},
   date      = {2019-06},
   doi       = {10.1109/JCDL.2019.00026},
   keywords  = {preprint,SchubotzCV},

src/biber/reformat.xml

@@ -609,11 +609,13 @@
       <field fieldtype="field" datatype="literal">review</field>
       <field fieldtype="field" datatype="literal">preprint</field>
       <field fieldtype="field" datatype="literal">biburl</field>
+      <field fieldtype="field" datatype="literal">core</field>
 
     </fields>
     <entryfields>
       <field>preprint</field>
       <field>biburl</field>
+      <field>core</field>
       <field>owner</field>
       <field>review</field>
       <field>abstract</field>