Refactor structure of review

Author: wendelinwemhoener

flashcard.txt

@@ -0,0 +1,2 @@
+package seedu.duke.flashcard.review;public class FlashcardReview {
+}

src/main/java/seedu/duke/flashcard/FlashcardReview.java

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+package seedu.duke.flashcard.reviewmode;public class RandomReview {
+}

src/main/java/seedu/duke/flashcard/review/FlashcardReview.java

@@ -1,3 +1,5 @@
+//@@author wendelinwemhoener
+
 package seedu.duke.flashcard;
 
 public enum FlashcardDifficulty {

src/main/java/seedu/duke/flashcard/review/RandomReviewMode.java

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+package seedu.duke.flashcard.reviewmode;public class ReviewMode {
+}

src/main/java/seedu/duke/flashcard/FlashcardDifficulty.java renamed to src/main/java/seedu/duke/flashcard/review/ReviewDifficulty.java

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+package seedu.duke.flashcard.reviewmode;public class SpacedRepetitionReview {
+}