Add CCE equations

- add tags for relevant CCE quantities
- add computation methods for CCE integrands

docs/References.bib

@@ -169,6 +169,37 @@ @book{Hartle2003gravity
   url        = "https://www.pearson.com/us/higher-education/program/Hartle-Gravity-An-Introduction-to-Einstein-s-General-Relativity/PGM298455.html",
 }
 
+@article{Bishop1997ik,
+  author         = "Bishop, Nigel T. and Gomez, Roberto and Lehner, Luis and
+                    Maharaj, Manoj and Winicour, Jeffrey",
+  title          = "{High powered gravitational news}",
+  journal        = "Phys. Rev.",
+  volume         = "D56",
+  year           = "1997",
+  pages          = "6298-6309",
+  doi            = "10.1103/PhysRevD.56.6298",
+  eprint         = "gr-qc/9708065",
+  archivePrefix  = "arXiv",
+  primaryClass   = "gr-qc",
+  url            = {https://arxiv.org/abs/gr-qc/9708065}
+}
+
+@article{Handmer2014qha,
+  author         = "Handmer, Casey J. and Szilagyi, B.",
+  title          = "{Spectral Characteristic Evolution: A New Algorithm for
+                    Gravitational Wave Propagation}",
+  journal        = "Class. Quant. Grav.",
+  volume         = "32",
+  year           = "2015",
+  number         = "2",
+  pages          = "025008",
+  doi            = "10.1088/0264-9381/32/2/025008",
+  eprint         = "1406.7029",
+  archivePrefix  = "arXiv",
+  primaryClass   = "gr-qc",
+  url            = {https://arxiv.org/abs/1406.7029}
+}
+
 @book{HesthavenWarburton,
   author     = "Hesthaven, Jan S. and Warburton, Tim",
   title      = "Nodal Discontinuous {Galerkin} Methods",