[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()