Adding Gnuplot writer for vector field.

Refactoring transport equation using the vector norm test function.

Refactoring the Lax-Fridrichs scheme for the transport equation.

 - sign
 - Heaviside
 - smooth Heaviside
 - shift
 - rotation

Curiously clang complained about it, but nvcc didn't.

The original algorithm used space as one of the separators between types
in template parameter lists, so as a result "A< short int >" was parsed
into a list of ("A", "short", "int").