Add Neighbour (NB) to output

[davidcsterratt]

As described in this PR #13

It is very useful to export the neighbor-relationship with the triangles.

I am trying to use the RTriangle Package in my PIHMgisR tool, that is a GIS-processing for Penn State Integrated Hydrologic Model (PIHM). After I build the triangle, it is necessary to define the neighbor-relationship among triangles for modeling.
I have my own code to build the topological relationship in R and C, but due to the algorithm, it is very time-consuming to build the relationship when the number of triangles become larger than larger. I notice the C version of the code already exports the neighbor-relationship, so it would be nice to update these two files and export the topological relationship explicitly.

[davidcsterratt]

I have committed the change. However, there are a couple of extra things needed:

• Documentation
• Fix output in tests, and ideally have new test for this