Fix \not vertical alignment (#1497)

* Fix \not vertical alignment

Fix #1491 by changing from `position: absolute` to `position: relative`.

* Switch to \rlap implementation of \not

* Separate \not the macro from \not the symbol via \@not

* Fix test

test/katex-spec.js

@@ -93,7 +93,11 @@ describe("A rel parser", function() {
             if (group.type === "htmlmathml") {
                 group = group.value.html[0];
             }
-            expect(group.type).toEqual("rel");
+            if (group.type === "mclass") {
+                expect(group.value.mclass).toEqual("mrel");
+            } else {
+                expect(group.type).toEqual("rel");
+            }
         }
     });
 });