Fix quadrilateral boundary segments in GmshReader

• Add support for quadrilaterals as boundary segments
• Throw an exception if unsupported boundary segment type is used. Otherwise segment insertion indices will be messed up even for supported segment types.
• Add test case with unit cube and quadrilateral boundary segments.

added bugfix label

mentioned in issue #115 (closed)

Notice that there's a few potential issues with this fix:

1. I never used Gmsh, so I'd prefer if someone else checks this.
2. It seems that quadrilateral boundary segments are only supported in UGGrid. However, looking at the code, UGGrid uses a 'linear' dummy parametrization if only the vertices are passed. This may fail for distorted cubes, unless 'linear' actually means (griddim-1)-linear.
3. The test just checks if quadrilateral segments can be inserted and not if the insertion order and tags are correct. But this may already fix the issue with boundary tags in #115 (closed).
4. With this patch GmshReader may throw for grids it accepted before. This is because it used to silently ignore unsupported boundary segments types which messed up the insertion indices. While I consider this a bugfix, it may annoy users if they have to fix their .msh files.

Looks good to me. Please merge. Consider adding a CHANGELOG entry.

As this is a clear fix and hardly invasive, I think this should be backported. And as the person who found the bug claims to still be on 2.6, we should consider a backport to there, too.

AFAIK UGGrid uses standard bilinear interpolation for quadrilateral boundary segments.

Disclaimer: I didn't check, but I really can't think why UGGrid should do something else.

resolved all threads

mentioned in commit 99142f99

merged

For the record: Another side effect of this is, that gmsh-files containing interior surface elements will lead to an exception. Such files have been accepted silently before although this is not supported by the GridFactory.