ApplyOnCustomBoundaryIntersections misses keep_alive in bindings

The pymor discretizer on this state results in a segfault since dd_grid.macro_based_boundary_info(I, macro_boundary_info) is directly given to the ApplyOnCustomBoundaryIntersections function. While this commit resolves the issue on the python side, it should also be solved in the bindings code. Some keep_alive functionality to the respective bindings did not work immediately though.