Test "Binary black hole GW150914" example

[Roland Haas]

Before each release, check that http://einsteintoolkit.org/gallery/bbh/index.html still works and produces correct output.

Keyword: None

[Samuel Cupp]

I ran the simulation using the master branch and compared the data with the data provided by the Gallery example. The comparison is available in an attachment on this ticket. During the 2018-08-27 weekly ET meeting, it was agreed that the differences were within reasonable tolerance.