message: Tweaks/optimizes `primal::Polyhedron::volume()`
  Inspired by PR suggestion.
diff: @@ -520,18 +520,18 @@ public:
  const PointType& origin = m_vertices[0];
  - for(int i = 0; i < face_count; i++)
  + for(int i = 0; i < face_count; ++i)
  {
  - int n = face_size[i];
  - VectorType v0 = m_vertices[faces[face_offset[i]]] - origin;
  + const int N = face_size[i];
  + const int i_offset = face_offset[i];
  + const VectorType v0 = m_vertices[faces[i_offset]] - origin;
  - for(int j = 1; j < n - 1; ++j)
  + for(int j = 1, k = 2; j < N - 1; ++j, ++k)
  {
  - const int k = (j + 1) < n ? j + 1 : 0;
  retVol += VectorType::scalar_triple_product(
  v0,
  - m_vertices[faces[face_offset[i] + j]] - origin,
  - m_vertices[faces[face_offset[i] + k]] - origin);
  + m_vertices[faces[i_offset + j]] - origin,
  + m_vertices[faces[i_offset + k]] - origin);
  }
  }
  }
  