collatz-conjecture: add start implementation. (#1021)

* collatz-conjecture: add start implementation.

* collatz-conjecture: update with @FridaTveit 's suggestion

exercises/collatz-conjecture/src/main/java/CollatzCalculator.java

+class CollatzCalculator {
+
+    int computeStepCount(int start) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}