fixes after merging PDELab upstream and feature/fast-dg-assembler

In detail:

• [flatmatrix] use class in forward decleration for Pattern because later it is defined so

• [flatmatrix,pattern] don't move local objects in return statement

  clang 3.8 warns:

  moving a local object in a return statement prevents copy elision [-Wpessimizing-move]

  return std::move(r);

• [ConvectionDiffusionDG] don't use Geometry::dimension in function setupCache It has been removed already.

• [LinearProblem,MatrixFree]

  • don't use BackendVectorSelector, this has been removed already
  • correct number types, similar to matrix-based SLP, for complex numbers