Add test and docs for DurationModifier

src/ext-test/java/org/opentripplanner/ext/flex/trip/DurationModifierTest.java

@@ -0,0 +1,20 @@
+package org.opentripplanner.ext.flex.trip;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.time.Duration;
+import org.junit.jupiter.api.Test;
+
+class DurationModifierTest {
+
+  @Test
+  void doesNotModify() {
+    assertFalse(DurationModifier.NONE.modifies());
+  }
+
+  @Test
+  void modifies() {
+    assertTrue(new DurationModifier(Duration.ofMinutes(1), 1.5f).modifies());
+  }
+}

src/ext/java/org/opentripplanner/ext/flex/trip/DurationModifier.java

@@ -5,12 +5,19 @@
 import org.opentripplanner.framework.time.DurationUtils;
 import org.opentripplanner.framework.tostring.ToStringBuilder;
 
+/**
+ * A modifier to influence the A*-calculated driving time of flex trips.
+ */
 public class DurationModifier implements Serializable {
 
   public static DurationModifier NONE = new DurationModifier(Duration.ZERO, 1);
   private final int offset;
   private final float factor;
 
+  /**
+   * @param offset A fixed offset to add to the driving time.
+   * @param factor A factor to multiply the driving time with.
+   */
   public DurationModifier(Duration offset, float factor) {
     if (factor < 0.1) {
       throw new IllegalArgumentException("Flex duration factor must not be less than 0.1");