Our views on the topics of this review have been shaped through discussions, interactions and collaborations, through the years and/or at the time of the writing of this review, with many colleagues, to whom we are greatly indebted. Among many others: James Bardeen, Horst Beyer, Luisa Buchman, David Brown, Gioel Calabrese, Peter Diener, Nils Dorband, Scott Field, Helmut Friedrich, Carsten Gundlach, Ian Hawke, Jan Hesthaven, Frank Herrmann, Larry Kidder, Oleg Korobkin, Heinz-Otto Kreiss, Luis Lehner, Lee Lindblom, Ken Mattsson, Vincent Moncrief, Gabriel Nagy, Darío Núñez, Pelle Olsson, Enrique Pazos, Harald Pfeiffer, Denis Pollney, Jorge Pullin, Christian Reisswig, Oscar Reula, Oliver Rinne, Milton Ruiz, Mark Scheel, Erik Schnetter, Magnus Svard, Bela Szilágyi, Eitan Tadmor, Nicholas Taylor, Saul Teukolsky, Jonathan Thornburg, James York, Jeffrey Winicour, Burkhard Zink, Anil Zenginoğlu.
We especially thank Stephen Lau, Oscar Reula, Eitan Tadmor and Anil Zenginoğlu for going through a previous version of this manuscript in great detail and providing us with very helpful feedback; and Liam MacTurk for very helpful edits.
OS thanks the Perimeter Institute for Theoretical Physics, where part of this work has been done, for its hospitality. MT thanks the Universidad Michoacana de San Nicolás de Hidalgo and Tryst DC Coffeehouse Bar and Lounge, where parts of this work were done, for their hospitality. This work has been partially supported by CONACyT Grant 61173, Grant CIC 4.19 to the Universidad Michoacana de San Nicolás de Hidalgo and by the National Science Foundation Grants PHY0801213 and PHY0908457 to the University of Maryland.
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