[AL] Extend SerialExecutor::execute argument list.

Now it accepts and forwards an arbitrary list of arguments.
[AL] Add SerialExecutor::execute for two container.

This is used in initialization of LocalAssemblerData for every mesh element.
[AL] Add LocalAssemblerBuilder.

It is a simplified version of the VectorMatrixAssembler.
Used in creating LocalAssemblerData from mesh elements.
[AL] Pass LocalAsm to VecMatAsm in operator().

Instead of saving a local assembler reference.
This also removes the local matrix/vector types
and some types are moved to the operator().
Update SerialLinearSolver/2dDiffusionQuad test.
[T] Add comment for empty assemble()
[T] Comment out unused function arguments.
[T] Replace LocalDataInitializer with function.

A static function in the SteadyDiffusion2DExample1 class.
[T] Move LocalAsmData into SteadyDiffusionExample.
[T] Remove unused type aliases.
Merge pull request #483 from endJunction/AssemblerLib

Assembler lib
[PL] Add mesh subset to initialize().
[PL] Initialize MeshSubsets and MeshComponentMap.
[PL] Rename mesh_items_all_nodes; items->subset.
[PL] Add destructor for GroundwaterFlowProcess.
[PL] Move mesh_component_map to class variables.

...from the initialize().
Also delete it in the dtor.
[PL] Add createDofMap() in Process.
[PL] Use number of node's DOF in createDOFMap().
[PL] Remove MeshComponentMap include from GW.
[PL] Add dof_map to GroundwaterFlow.