Merge branch 'develop' of ssh://git.mpi-cbg.de/openfpm/openfpm_numerics into develop

src/BoundaryConditions/MethodOfImages.hpp

...yConditions/tests/method_of_images_cylinder_unit_test.cpp

src/BoundaryConditions/tests/method_of_images_unit_test.cpp

src/CMakeLists.txt

src/FiniteDifference/FD_simple.hpp

src/FiniteDifference/Upwind_gradient.hpp

src/FiniteDifference/tests/FD_simple_unit_test.cpp

src/level_set/redistancing_Sussman/RedistancingSussman.hpp