Simplify populateNeighborhood by eliminating redundant operations

[erdenbatuhan]

Please verify and fix the following:

• Unless I'm missing a special case here, we don't need to check the duplicate neighbors because we iterate over all neighbors for each vertex, and each face is processed only once for each vertex. We can set the map entry directly instead.
  

  interactive-arap/src/Arap.cpp

  Line 45 in b864ab7

  for (auto neighbor: allNeighbors) { // Iterate over all the found neighbors of the vertex

• Since the innermost if-condition will only be true once per face, we can ideally exit the innermost loop when a match is found.
  

  interactive-arap/src/Arap.cpp

  Line 38 in b864ab7

  if (faces(j, k) == i) {