Fix missing Update of structural nodes in Solver::Eig needed for automatic differentiation.

This branch contains several fixes and improvements for the Arpack based eigen-solver and the multi-threaded data manager. Now it is possible to use the same linear solvers and matrix handlers for the Arpack based eigen-solver which are used by the nonlinear solver. In addition to that, the accuracy of computed eigenvalues of preloaded and curved beam structures can be improved in some cases thanks to automatic differentiation.