[license] unified url and fixed some minor typos

dune/stuff/functions.hh

dune/stuff/functions/ESV2007.hh

dune/stuff/functions/checkerboard.hh

dune/stuff/functions/combined.hh

dune/stuff/functions/constant.hh

dune/stuff/functions/default.hh

dune/stuff/functions/expression.hh

dune/stuff/functions/expression/base.hh

dune/stuff/functions/femadapter.hh

dune/stuff/functions/global.hh

dune/stuff/functions/interfaces.hh

dune/stuff/functions/spe10.hh

dune/stuff/test/functions_checkerboard.cc

dune/stuff/test/functions_constant.cc

dune/stuff/test/functions_expression.cc

dune/stuff/test/functions_functions.cc