[assembler.gridwalker] let the gridwalker itself be a functor
[operators.base] replaced AssemblableVolumeBase by MatrixBasedBase
[operators.elliptic] based EllipticCG on SystemAssembler
[operators.base] MatrixBasedBase -> MatrixBased
[operators.elliptic] update
[functionals.base] AssemblableVolumeBase -> VectorBased
[functionals.l2] based L2Volume on SystemAssembler
[operators] elliptic -> elliptic-cg
[tests.elliptic-cg...] update
[assembler.gridwalker] expand GridWalkerWrapper
[functionals.base] removed AssemblableFaceBase
[functionals.l2] based L2Face on SystemAssembler