Make report curve units more flexible/optional

Decouple X and Y unit requirements for the curve schema.
This allows mixing values with and without units.
An example case is probability (unitless) vs. time (hours).

With the coupled schema,
both X and Y must or must not have units.
The above example would fail to validate.

mef/schema/curves.rnc

@@ -5,14 +5,14 @@ curve =
     (attribute X-title { xsd:string },
      attribute Y-title { xsd:string },
      attribute Z-title { xsd:string }?)?,
-    (attribute X-unit { unit },
-     attribute Y-unit { unit },
-     attribute Z-unit { unit }?)?,
+    attribute X-unit { unit }?,
+    attribute Y-unit { unit }?,
+    attribute Z-unit { unit }?,
     element point {
       attribute X { xsd:double },
       attribute Y { xsd:double },
       attribute Z { xsd:double }?
     }*
   }
 
-unit = "seconds" | "hours"
+unit = "seconds" | "hours" | "seconds-1" | "hours-1" | "years" | "years-1"