add single black hole example to gallery

[Roland Haas]

There is currently no example in the gallery on how to run a single black hole. While multiple tests use single black hole initial data, these often start with Kerr-Schild initial data and not the more common Bowen-York initial data which can be easily extended to binary black hole settings.

The Kerr example in the original ET paper Loeffler et al. http://arxiv.org/abs/1111.3344 is available in EinsteinExamples (checked out into the “par” directory) as by-a0.7.par and could be adapted.