[Cleanup] Restructure with subdirectories

src/CMakeLists.txt

src/hdf5/iteration-writer.hh

src/program_state.hh

src/sand-wedge.cc

src/spatial-solving/fixedpointiterator.cc

src/spatial-solving/fixedpointiterator.hh

src/spatial-solving/fixedpointiterator_tmpl.cc

src/spatial-solving/solverfactory.cc

src/spatial-solving/solverfactory.hh

src/spatial-solving/solverfactory_tmpl.cc

src/time-stepping/adaptivetimestepper.cc

src/time-stepping/adaptivetimestepper.hh

src/time-stepping/adaptivetimestepper_tmpl.cc

src/time-stepping/coupledtimestepper.cc

src/time-stepping/coupledtimestepper.hh

src/time-stepping/coupledtimestepper_tmpl.cc

src/time-stepping/rate.cc

src/time-stepping/rate.hh

src/time-stepping/rate/backward_euler.cc

src/time-stepping/rate/backward_euler.hh