Add type annotation required by rustc 2015-03-31

Resolves this error, which is fallout from
rust-lang/rust#23673:

src/structs/dmat.rs:501:43: 501:74 error: type annotations required: cannot resolve `<f64 as core::ops::Div<_>>::Output == f64` [E0284]
src/structs/dmat.rs:501         let normalizer: N    = Cast::from(1.0f64 / Cast::from(self.nrows));
                                                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Anders Kaseorg <[email protected]>

Author: andersk

src/structs/dmat.rs

@@ -498,7 +498,8 @@ impl<N: Clone + Copy> Transpose for DMat<N> {
 impl<N: BaseNum + Cast<f64> + Clone> Mean<DVec<N>> for DMat<N> {
     fn mean(&self) -> DVec<N> {
         let mut res: DVec<N> = DVec::new_zeros(self.ncols);
-        let normalizer: N    = Cast::from(1.0f64 / Cast::from(self.nrows));
+        let nrows_f64: f64   = Cast::from(self.nrows);
+        let normalizer: N    = Cast::from(1.0f64 / nrows_f64);
 
         for i in 0..self.nrows {
             for j in 0..self.ncols {