paper.bib: add doi or url for all refs

paper.bib

@@ -4,15 +4,16 @@ @phdthesis{heeneMassivelyParallelCombination2018
   title = {A {{Massively Parallel Combination Technique}} for the {{Solution}} of {{High-Dimensional PDEs}}},
   author = {Heene, Mario},
   date = {2018},
-  institution = {{Institut für Parallele und Verteilte Systeme der Universität Stuttgart}},
+  doi = {10.18419/opus-9893},
+  institution = {{Universität Stuttgart}},
   keywords = {combination technique,HPC,plasma physics,sparse grids}
 }
 
 @phdthesis{obersteiner_spatially_2021,
   title       = {A spatially adaptive and massively parallel implementation of the fault-tolerant combination technique},
   url         = {https://mediatum.ub.tum.de/doc/1613369/1613369.pdf},
   institution = {Technische Universit\"at M\"unchen},
-  type        = {Dissertation},
+  type        = {phdthesis},
   author      = {Obersteiner, Michael},
   date        = {2021}
 }
@@ -37,7 +38,7 @@ @article{pollingerStableMassconservingSparse2023
   author = {Pollinger, Theresa and Rentrop, Johannes and Pflüger, Dirk and Kormann, Katharina},
   date = {2023-07-07},
   journaltitle = {Journal of Computational Physics},
-	shortjournal = {JCOMP},
+  shortjournal = {JCOMP},
   pages = {112338},
   issn = {0021-9991},
   doi = {10.1016/j.jcp.2023.112338},