Last Finite Differences not working required several other components

src/FiniteDifference/Derivative.hpp

src/FiniteDifference/FDScheme.hpp

src/FiniteDifference/FDScheme_unit_tests.hpp

src/FiniteDifference/eq.hpp

src/FiniteDifference/eq_unit_test.hpp

src/FiniteDifference/sum.hpp