Add sliding mesh capability

CHANGELOG.md

docs/user-guide/utilities/nekmesh.tex

docs/user-guide/xml/xml-filters.tex

docs/user-guide/xml/xml-movement.tex

library/FieldUtils/OutputModules/OutputVtk.cpp

library/MultiRegions/AssemblyMap/InterfaceMapDG.cpp

library/MultiRegions/AssemblyMap/InterfaceMapDG.h

library/MultiRegions/ExpList.cpp

library/MultiRegions/ExpList.h

library/NekMesh/CMakeLists.txt

library/NekMesh/MeshElements/Node.h

library/NekMesh/Module/InputModules/InputNekpp.cpp

library/NekMesh/Module/OutputModules/OutputNekpp.cpp

library/NekMesh/Module/ProcessModules/ProcessCombine.cpp

library/NekMesh/Module/ProcessModules/ProcessCombine.h

library/SolverUtils/ALEHelper.cpp

library/SolverUtils/ALEHelper.h

library/SolverUtils/Advection/AdvectionWeakDG.cpp

library/SolverUtils/Advection/AdvectionWeakDG.h

library/SolverUtils/CMakeLists.txt