Small changes in I/O and data type management

src/ADerrorsIO.jl

@@ -36,6 +36,9 @@ println("a has error: ", err(a))
 ```
 """
 function err(a::uwreal)
+    if (length(a.prop) == 0)
+        return 0.0
+    end
     if (length(a.cfd) == 0)
         error("No error available... maybe run uwerr")
     end

src/ADerrorsTypes.jl

@@ -108,3 +108,7 @@ function Base.show(io::IO, a::uwreal)
         print(io, a.mean, " (Error not available... maybe run uwerr)")
     end
 end
+
+Base.zero(u::uwreal) = uwreal(0.0)
+Base.one(u::uwreal)  = uwreal(1.0)
+Base.one(u::Type{uwreal})  = uwreal(1.0)