Compatibility fix: in geometric centering, use @rdivide instead of @mrdivide

@rdivide is actually more correct, because we’re not doing an
element-by-element division.

And, more importantly, MATLAB R2009b does not accept @mrdivide in bsxfun.

src/@dseries/center_.m

@@ -33,7 +33,7 @@ end
 m = mean(o.data);
 
 if geometric
-    o.data = bsxfun(@mrdivide, o.data, m);
+    o.data = bsxfun(@rdivide, o.data, m);
 else
     o.data = bsxfun(@minus, o.data, m);
 end