Adding Gnuplot writer for vector field.

Refactoring transport equation using the vector norm test function.

Refactoring the Lax-Fridrichs scheme for the transport equation.

This reverts commit 40379dcf.

Apart from squashing begin, end, entityOrientation and entityBasis
parameters into TraverserKernelData, this does not improve performance
of the LinearSystemAssembler, ExplicitUpdater etc., but in tnl-mhfem it
allows us to pass MeshDependentData, which is already available as a
SharedPointer, directly to the grid traverser without duplicating the
transfer to GPU.