fix: abs works with WHOLE interval (fixes #26)

Author: mauriciopoppe

src/operations/misc.test.ts

@@ -123,6 +123,8 @@ describe('Interval', function () {
     Interval.almostEqual(n, [2, 3])
     n = Interval.abs(new Interval(2, 3))
     Interval.almostEqual(n, [2, 3])
+    n = Interval.abs(Interval.WHOLE)
+    Interval.almostEqual(n, Interval.EMPTY)
   })
 
   it('should compute the max value of two intervals', function () {

src/operations/misc.ts

@@ -309,7 +309,7 @@ export const wid = width
  * @return {Interval}
  */
 export function abs(x: Interval): Interval {
-  if (utils.isEmpty(x)) {
+  if (utils.isEmpty(x) || utils.isWhole(x)) {
     return constants.EMPTY
   }
   if (x.lo >= 0) {