Move assembler-files into a separate assembler folder

CHANGELOG.md

dune/gfe/CMakeLists.txt

dune/gfe/assemblers/CMakeLists.txt

dune/gfe/assemblers/chiralskyrmionenergy.hh

dune/gfe/assemblers/cosseratenergystiffness.hh

dune/gfe/assemblers/cosseratrodenergy.hh

dune/gfe/assemblers/geodesicfeassembler.hh

dune/gfe/assemblers/geodesicfeassemblerwrapper.hh

dune/gfe/assemblers/harmonicenergy.hh

dune/gfe/assemblers/l2distancesquaredenergy.hh

dune/gfe/assemblers/localenergy.hh

dune/gfe/assemblers/localfirstordermodel.hh

dune/gfe/assemblers/localgeodesicfeadolcstiffness.hh

dune/gfe/assemblers/localgeodesicfefdstiffness.hh

dune/gfe/assemblers/localgeodesicfestiffness.hh

dune/gfe/assemblers/localintegralenergy.hh

dune/gfe/assemblers/mixedgfeassembler.hh

dune/gfe/assemblers/mixedlocalgeodesicfestiffness.hh

dune/gfe/assemblers/mixedlocalgfeadolcstiffness.hh

dune/gfe/assemblers/nonplanarcosseratshellenergy.hh