Add missing @NotNull annotations

Author: Donnerbart

src/main/java/de/donnerbart/split/FormatOption.java

@@ -14,7 +14,7 @@ public enum FormatOption {
     }
 
     @Override
-    public String toString() {
+    public @NotNull String toString() {
         return parameterValue;
     }
 }

src/main/java/de/donnerbart/split/NewTestTimeOption.java

@@ -16,7 +16,7 @@ public enum NewTestTimeOption {
     }
 
     @Override
-    public String toString() {
+    public @NotNull String toString() {
         return parameterValue;
     }
 }

src/main/java/de/donnerbart/split/model/Split.java

@@ -83,7 +83,7 @@ public int hashCode() {
     }
 
     @Override
-    public String toString() {
+    public @NotNull String toString() {
         return "Split{" +
                 "index=" +
                 formatIndex() +