Assembler lib2

I would like to present you the remaining two parts of AssemblerLib initially proposed by Norihiro.

Both parts are independent of each other, but not the test of the LinearEquationAssembler.

The first part consists of a SerialVectorMatrixBuilder for global vector/matrix construction, and a function in SerialExecutor file for index-aware for-each loop.

The second part is a LinearEquationAssembler. It is tested by solving a steady-state 2D diffusion problem on a quad mesh.