Move stepper and main files to fem-dg, goodbye dgtest.*

configure.ac

dune/fem-dg/Makefile.am

dune/fem-dg/main/Makefile.am

dune/fem-dg/main/codegen.hh

dune/fem-dg/main/main.cc

dune/fem-dg/main/main.hh

dune/fem-dg/main/main_0.cc

dune/fem-dg/main/main_1.cc

dune/fem-dg/main/main_2.cc

dune/fem-dg/main/main_3.cc

dune/fem-dg/main/main_4.cc

dune/fem-dg/main/main_pol.cc

dune/fem-dg/misc/Makefile.am

dune/fem-dg/misc/cons2prim.hh

dune/fem-dg/stepper/Makefile.am

dune/fem-dg/stepper/advectionstepper.hh

dune/fem-dg/stepper/base.hh

dune/fem-dg/stepper/baseevolution.hh

dune/fem-dg/stepper/ellipt.hh

dune/fem-dg/stepper/stepper.hh