Small refactor

src/FiniteDifference/FDScheme.hpp

src/FiniteDifference/FDScheme_unit_tests.hpp

src/FiniteDifference/eq.hpp

src/FiniteDifference/eq_unit_test.hpp

src/FiniteDifference/eq_unit_test_3d.hpp

src/FiniteDifference/mul.hpp

src/Makefile.am

src/interpolation/interpolation.hpp

src/util/grid_dist_testing.hpp

src/util/util_num.hpp

src/util/util_num_unit_tests.hpp