Llama/Coordinates: Add "Cartesian planar" (2d) domain

[Erik Schnetter]

This offers a true 2d domain. It’s very similar to the regular Cartesian (3d) domain, but can handle grids that are only a single point wide in the 3rd direction. This is useful for axisymmetric simulations that don’t need Cartoon.

https://bitbucket.org/llamacode/llama/pull-requests/6/coordinates-add-cartesian-planar-2d-domain

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[Roland Haas]

Not having looked at the code: this should have a test case to be eligible for inclusion.

[Roland Haas]

Please add a description of the new coordinate system to Llama’s arrangement docs: https://bitbucket.org/llamacode/llama/src/master/doc/documentation.tex visible on the web http://einsteintoolkit.org/arrangementguide/Llama/documentation.html