Apparently there is an abs function defined in <cstdlib>, which works
only on integers. We want the std::abs overloads defined in <cmath>,
which are used by TNL::abs.

clang rocks!

http://stackoverflow.com/questions/29136607/c-what-is-wrong-with-abs#29136774

Adding Gnuplot writer for vector field.

The methods did not work with zero array size, so extra checks were
added.

Refactoring transport equation using the vector norm test function.

Refactoring the Lax-Fridrichs scheme for the transport equation.