Adding missing files

src/FiniteDifference/Derivative.hpp

src/FiniteDifference/FDScheme.hpp

src/FiniteDifference/FDScheme_unit_tests.hpp

src/FiniteDifference/Laplacian.hpp

src/FiniteDifference/System.hpp

src/FiniteDifference/eq.hpp

src/FiniteDifference/eq_unit_test.hpp

src/FiniteDifference/sum.hpp

src/FiniteDifference/util/common.hpp

src/FiniteDifference/util/common_test.hpp