Don't use gray for unit decompositions (#13)

Despite being an official SI derived unit, gray is only meant to be used
in the context of ionizing radiation. There is no official SI unit for
specific heat capacity.

core/src/loader/load.rs

@@ -379,7 +379,6 @@ pub(crate) fn load_defs(ctx: &mut Context, defs: Defs) -> Vec<String> {
     decomposition_units.insert("tesla");
     decomposition_units.insert("lumen");
     decomposition_units.insert("lux");
-    decomposition_units.insert("gray");
     decomposition_units.insert("katal");
 
     let mut prefix_lookup = BTreeMap::new();

core/tests/query.rs

@@ -351,7 +351,7 @@ fn test_unary_operators() {
 
 #[test]
 fn test_equals() {
-    test("a = kg N / W^2", "1 second^2 / gray meter");
+    test("a = kg N / W^2", "1 second^4 / meter^3");
     test(
         "1 = kg",
         "= is currently only used for inline unit definitions: expected unit, got 1",