Given a function X, return a function that takes two points A, B and computes the approximation of the area under the curve of F(X)
1> c(riemann).
{ok,riemann}
2> F = riemann:riemann(fun(X) -> 1 end, 0.1).
#Fun<riemann.2.110299938>
3> F(0, 0.9).
0.9999999999999999