port openstructure dependency from Eigen2 to Eigen3

changes:

-GetPrincipalAxes in Vec3List
 Results with Eigen3 have been verified with 10 randomly picked pdb structures.
 Differences only occur with changing signs of the vectors.

-CalcPrincipalAxis in BoundingBox
 No difference in algorithm, the SelfAdjointEigenSolver no lives in
 Eigenvalues. Does in principal the same thing as the function above,
 just using different functionality.

-SVD superposer
 Unit tests still run through with the changes introduced for Eigen3
 and also manual testing seems to work well.

-CartoonRenderer
 Principal axes calculation for rendering helices as cylinders had to be
 adapted. Rendering looks exactly the same...

-SuperposeFrames
 Not yet tested...

ToDo:
numeric.h and levenberg_marquardt.h in the img/alg module have to be
ported. This will be done in the next few days.