get_boundary()

[ggorman]

Added get_bounday feature to c interface and Mesh class.

[ggorman]

Can you merge with master and I'll review it for merging please?

[taupalosaurus]

I have left a few questions in include/Mesh.h (marked with "TODO") that must be solved before merging:

• when you get the boundary, do you do something complicated to preserve the orientation of the surface
• why not do it in 2d then ?
• I think the indices j+... are wrong (j+0 is the current node with a tag>0, it's not in the facet?)