Add shipping containers as a relative unit of volume

macrovision.js

@@ -158,6 +158,10 @@ math.createUnit("people", {
     definition: "75 liters",
     prefixes: "long"
 });
+math.createUnit("shippingContainers", {
+    definition: "1169 ft^3",
+    prefixes: "long"
+});
 math.createUnit("olympicPools", {
     definition: "2500 m^3",
     prefixes: "long"
@@ -366,6 +370,7 @@ const unitChoices = {
         ],
         "relative": [
             "people",
+            "shippingContainers",
             "olympicPools",
             "oceans",
             "earthVolumes",