restore allocation free maxdiff

The mapreduce based version allocates which is undesirable.
This change restores the previous allocation free code for normal Arrays while still allowing gpu arrays using mapreduce

src/utilities/maxdiff.jl

@@ -1,3 +1,16 @@
+# generic version for gpu support
 function maxdiff(x::AbstractArray, y::AbstractArray)
     return mapreduce((a, b) -> abs(a - b), max, x, y)
 end
+
+# allocation free version for normal arrays
+function maxdiff(x::Array, y::Array)
+    res = real(zero(x[1] - y[1]))
+    @inbounds for i in 1:length(x)
+        delta = abs(x[i] - y[i])
+        if delta > res
+            res = delta
+        end
+    end
+    return res
+end