Change mean_axis to use FromPrimitive

The documentation for the `Zero` and `One` traits says only that they
are the additive and multiplicative identities; it doesn't say
anything about converting an integer to a float by adding `One::one()`
to `Zero::zero()` repeatedly. Additionally, it's nice to convert the
length to `A` directly instead of having to use a loop.

Author: jturner314

src/numeric/impl_numeric.rs

@@ -7,7 +7,7 @@
 // except according to those terms.
 
 use std::ops::{Add, Div, Mul};
-use libnum::{self, One, Zero, Float};
+use libnum::{self, Zero, Float, FromPrimitive};
 use itertools::free::enumerate;
 
 use imp_prelude::*;
@@ -112,8 +112,9 @@ impl<A, S, D> ArrayBase<S, D>
 
     /// Return mean along `axis`.
     ///
-    /// **Panics** if `axis` is out of bounds or if the length of the axis is
-    /// zero and division by zero panics for type `A`.
+    /// **Panics** if `axis` is out of bounds, if the length of the axis is
+    /// zero and division by zero panics for type `A`, or if `A::from_usize()`
+    /// fails for the axis length.
     ///
     /// ```
     /// use ndarray::{aview1, arr2, Axis};
@@ -126,16 +127,12 @@ impl<A, S, D> ArrayBase<S, D>
     /// );
     /// ```
     pub fn mean_axis(&self, axis: Axis) -> Array<A, D::Smaller>
-        where A: Clone + Zero + One + Add<Output=A> + Div<Output=A>,
+        where A: Clone + Zero + FromPrimitive + Add<Output=A> + Div<Output=A>,
               D: RemoveAxis,
     {
-        let n = self.len_of(axis);
+        let n = A::from_usize(self.len_of(axis)).expect("Converting axis length to `A` must not fail.");
         let sum = self.sum_axis(axis);
-        let mut cnt = A::zero();
-        for _ in 0..n {
-            cnt = cnt + A::one();
-        }
-        sum / &aview0(&cnt)
+        sum / &aview0(&n)
     }
 
     /// Return variance along `axis`.