Mortar segmentation bugs

[ahojukka5]

Added 4 new tests, all are failing (see https://github.com/JuliaFEM/JuliaFEM.jl/blob/bug/mortar_discretization/test/test_problems_mortar_3d.jl and https://travis-ci.org/JuliaFEM/JuliaFEM.jl/builds/196842417)

A simple block model with interface between upper and lower block:

Set temperature boundary condition T=0.0 to top of upper block and T=1.0 to bottom of lower block. What should be expected is that we have a constant temperature T=0.5 at the interface as Mortar method should be able to deliver constant field over non-conforming interface without error.

• Tet4 elements: UMFPACK gives error, maybe polygons with zero are in interface, discretization of interface fails
• Tet10 elements, quadratic surface elements: interface area != 1.0, discretization of interface fails
• Tet10 elements, linear surface elements: temperature in interface not constant 0.5
• Tet10 elements, quadratic surface elements are splitted to smaller linear sub-elements: temperature in interface not constant 0.5

[ahojukka5]

This is now fixed, see PR #88.