ContactMerge: store intersection node types

When solving large deformation contact problems, one needs to linearise the contact projection w.r.t the deformation. This requires to know how each remote intersection node was created, i.e.

1. Forward projected grid1 vertex
2. Inverse projected grid2 vertex
3. Edge intersection
4. Center of mass of intersection polygon, added to triangulate.

According to this, different linearisation formulas apply. In this branch I added a new member to ContactMerge which is storing this additional information and (hopefully) keeping it consistent, even when used within the advancing front algorithm (I will soon provide a test for this).