COMP: fix comp errors due to rebase

After rebasing on dynamics-sprint, the following
errors appeared:
- change from VecNf to VectorF
- wrongfully placed 'virtual' in MassSpringForceModel
- wrongfully placed '#endif' in MassSpringForceModel
- missing include in NonLinearSolver
- error in constructor in NonLinearSystem
- error in Eigen include in TimeIntegrator
- error in VTK remote address in External_VTK.cmake