more optimisations, range loops, typos, etc.

The length was used only internaly and
can be computed on the fly. This makes the
cumbersome and errorprone update of the length
unnecessary.

This allows to use MeshLib::Node and GL::Point in
the changed methods.

Using Eigen::Vector3d allows some nice shortening
of the code.

- access the minimal / maximal point of the AABB
  via public interface functions.
- Using auto for minimal / maximal point to
  get independent from the point type.

- don't modify the bounding box in SurfaceGrid
- access the minimal / maximal point of the AABB
  via public interface functions.
- Using auto for minimal / maximal point to
  get independent from the point type.