implemented new linear solver approach + deactivated nin_2d_laplace

. the nin_2d_laplace example does not generate reasonable results
  switched the example to be a linear problem.
  output looks like, that the solution is not written back from the
  linear solver. The linear solver (ViennaCL) in fact reports
  an arbitrary high numer and error values -> indicating a problem
  during the solving step.
  example also did not work at the last commit, being:
  70e65700b7e94c48b47f5cf63aa86ca1ab1b4b5b

. instead of using a ViennaCL-based 'solve' method, I switched
  to a linear solver functor approach. Currently a ViennaCL
  class  is available allowing to set all kinds of solvers and preconditioners.
  One forwards a configured linear solver object to the 'pde_solver' object,
  which then makes use of it.
  -> We can use this to proivde also a Trilinos/PETSc linear solver object.
  Also we can extend this approach for nonlinear solver, so we have a
  gummel and newton class.

. added additonal debug output during the nonlinear iteration,
  this way we are notified whether, e.g., the linear solver converged during a step..

== except of mosfet_3d and nin_2d_laplace - all examples execute and produce reasonable results