ellipt demo with convergence study added

.gitmodules

CMakeLists.txt

examples/CMakeLists.txt

examples/ellipt.cc

examples/init/ellipt.dat.2d

examples/init/ellipt.dat.3d

externals/CMakeLists.txt

externals/fmt

src/amdis/DirichletBC.hpp

src/amdis/GridFunctionOperator.hpp

src/amdis/common/Math.hpp

src/amdis/linear_algebra/LinearSolverInterface.hpp

src/amdis/linear_algebra/mtl/DOFMatrix.hpp

src/amdis/linear_algebra/mtl/DOFVector.hpp