Improving the documentation of numeric

src/FiniteDifference/Average.hpp

src/FiniteDifference/Derivative.hpp

src/FiniteDifference/FDScheme.hpp

src/FiniteDifference/FDScheme_unit_tests.hpp

src/FiniteDifference/Laplacian.hpp

src/FiniteDifference/eq.hpp

src/FiniteDifference/eq_unit_test.hpp

src/FiniteDifference/mul.hpp

src/FiniteDifference/sum.hpp

src/Makefile.am

src/Vector/Vector_eigen_util.hpp