Fixes on AMG + documentation + AMG report

src/FiniteDifference/Derivative.hpp

src/FiniteDifference/FDScheme_unit_tests.hpp

src/FiniteDifference/Laplacian.hpp

src/FiniteDifference/eq_unit_test_3d.hpp

src/Makefile.am

src/Matrix/SparseMatrix_petsc.hpp

src/Solvers/petsc_solver.hpp

src/Solvers/umfpack_solver.hpp

src/Vector/Vector_petsc.hpp

src/util/petsc_util.hpp