Add details of the reference paper in the CITATION.cff file

CITATION.cff

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+cff-version: "1.2.0"
+authors:
+- family-names: Leuschner
+  given-names: Matthias
+  orcid: "https://orcid.org/0000-0003-0477-3441"
+- family-names: Fritzen
+  given-names: Felix
+  orcid: "https://orcid.org/0000-0003-4926-0068"
+preferred-citation:
+  authors:
+  - family-names: Leuschner
+    given-names: Matthias
+    orcid: "https://orcid.org/0000-0003-0477-3441"
+  - family-names: Fritzen
+    given-names: Felix
+    orcid: "https://orcid.org/0000-0003-4926-0068"
+  date-published: 2017-11-30
+  doi: 10.1007/s00466-017-1501-5
+  issn: 1432-0924
+  issue: 3
+  journal: Computational Mechanics
+  publisher:
+    name: Springer
+  title: "Fourier-Accelerated Nodal Solvers (FANS) for homogenization problems"
+  type: article
+  url: "https://link.springer.com/article/10.1007/s00466-017-1501-5"
+  volume: 62
+title: "Fourier-Accelerated Nodal Solvers (FANS) for homogenization problems"