Create CITATION.bib (#43)

CITATION.bib

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+@online{ranocha2021adaptive,
+  title={Adaptive numerical simulations with {T}rixi.jl:
+         {A} case study of {J}ulia for scientific computing},
+  author={Ranocha, Hendrik and Schlottke-Lakemper, Michael and Winters, Andrew Ross
+          and Faulhaber, Erik and Chan, Jesse and Gassner, Gregor},
+  year={2021},
+  month={08},
+  eprint={2108.06476},
+  eprinttype={arXiv},
+  eprintclass={cs.MS}
+}
+
+@article{schlottkelakemper2021purely,
+  title={A purely hyperbolic discontinuous {G}alerkin approach for
+         self-gravitating gas dynamics},
+  author={Schlottke-Lakemper, Michael and Winters, Andrew R and
+          Ranocha, Hendrik and Gassner, Gregor J},
+  journal={Journal of Computational Physics},
+  pages={110467},
+  year={2021},
+  month={06},
+  publisher={Elsevier},
+  doi={10.1016/j.jcp.2021.110467},
+  eprint={2008.10593},
+  eprinttype={arXiv},
+  eprintclass={math.NA}
+}