Cell orientation preservation under dim=2 and spacedim=3

[jihuan-tian]

1. Modified convert_triangles and change_vertices_order for preserving the orientations of quadrangular cells, under the case dim=2 and spacedim=3. The consistency of orientations between the parent triangle and children quadrangles is evaluated by checking cell normal vectors instead of 2D cell measure, which cannot be used in this case.

2. Add .clang-format.

[martemyev]

Hi Jihuan,

Thanks for the PR! While I appreciate the clang-format changes and I fully support enforcing one code style for future projects, I do not think that it is warranted for this project. Hence I would prefer not to change that.

Regarding the preservation of the orientation of quadrangular cells - could you please create an issue for that, so that it could be more clear what the problem is? Then a PR containing the fix (with tests) for that particular issue would be more than welcome.

Thanks,
Mikhail