[test.la_container] added
[la.container.istl] be friendly to the solvers
[la.container.istl] give better compile errors
[la.solver.interface] whitespace
[la.solver.istl] added first draft
* BicgstabILUTSolver seems to work,
  but slower than the eigen one
* AmgSolver seems to work,
  faster than eigen if > 3*10^6 DoFs