[Compilation fix] Ensure proper aligned_allocator definition by using vector-aligned

[hrobeers]

https://github.com/jbaayen/vortexje/blob/master/vortexje/solver.cpp#L1003 Fails to compile in recent eigen releases due to improper allocator definition.

A simple fix could be replacing:

vector<vector<Vector3d, Eigen::aligned_allocator<Vector3d> >, Eigen::aligned_allocator<vector<Vector3d> > > wake_velocities;

with:

vector<vector<Vector3d, Eigen::aligned_allocator<Vector3d> >, Eigen::aligned_allocator<vector<Vector3d, Eigen::aligned_allocator<Vector3d> > > > wake_velocities;

However, a more readable and less error prone solution is proposed by this pull request with the introduction of vector_aligned:

  template <typename T>
  using vector_aligned = std::vector<T, Eigen::aligned_allocator<T> >;