Handle unused boundary nodes correctly

Ansgar Burchardtrequested to merge
bugfix/issue-55-uggrid-boundary-segments into master

For "unused" boundary nodes the special value -1 is used. Mapping this through isBoundaryNode[-1] would result in an out-of-bounds memory access and invalid values in the vertices_c_style array.

Closes: #54 (closed), #55 (closed)

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