Add U (server rack height) as a length unit

macrovision.js

@@ -97,6 +97,11 @@ math.createUnit("fathoms", {
     aliases: ["fathom", "fathoms"]
 });
 
+math.createUnit("U", {
+    definition: "1.75 inches",
+    prefixes: "short"
+});
+
 math.createUnit("earths", {
     definition: "12756km",
     prefixes: "long",
@@ -415,6 +420,7 @@ const unitChoices = {
             "smoots",
             "furlongs",
             "HUs",
+            "U",
             "fathoms",
         ],
         "human": [