Fix various issues with some problems

- where files are required from the "starter" (./src/main/java), these
  are now soft-linked into the example source.
- a couple examples were missing import statements.
- the example code for Hamming was failing the test suite.

At this time, these are all of the problems that have example code.

allergies/src/example/java/Allergen.java

+../../main/java/Allergen.java

gigasecond/src/example/java/Gigasecond.java

+import java.time.LocalDateTime;
+import java.time.LocalDate;
+
 public class Gigasecond {
 
     private LocalDateTime birthDateTime;

hamming/src/example/java/Hamming.java

 public class Hamming {
 
     public static int compute(String leftStrand, String rightStrand) {
+        if(leftStrand.length() != rightStrand.length()) {
+            throw new IllegalArgumentException("leftStrand and rightStrand must be of equal length.");
+        }
         final int length = Math.min(leftStrand.length(), rightStrand.length());
         int distance = 0;
         for (int i = 0; i < length; i++) {

meetup/src/example/java/MeetupSchedule.java

+../../main/java/MeetupSchedule.java

roman-numerals/src/example/java/RomanNumeral.java

+import java.util.Map;
+import java.util.LinkedHashMap;
+
 public class RomanNumeral {
 
     private int arabicNumeral;

scrabble-score/src/example/java/Scrabble.java

+import java.util.Map;
+import java.util.HashMap;
+
 public class Scrabble {
 
     private String word;

settings.gradle

 include 'accumulate'
-// include 'allergies'
+include 'allergies'
 include 'anagram'
 include 'atbash-cipher'
 include 'binary'
 include 'bob'
 include 'crypto-square'
 include 'etl'
-// include 'gigasecond'
+include 'gigasecond'
 include 'grade-school'
-// include 'hamming'
+include 'hamming'
 include 'luhn'
-// include 'meetup'
-// include 'nucleotide-count'
+include 'meetup'
+include 'nucleotide-count'
 include 'octal'
 include 'pascals-triangle'
 include 'phone-number'
 include 'raindrops'
 include 'rna-transcription'
 include 'robot-name'
-// include 'roman-numerals'
+include 'roman-numerals'
 include 'prime-factors'
-// include 'scrabble-score'
+include 'scrabble-score'
 include 'sieve'
 include 'simple-cipher'
 include 'space-age'
 include 'strain'
-// include 'triangle'
+include 'triangle'
 include 'trinary'
 include 'word-count'

triangle/src/example/java/Triangle.java

+import java.util.HashSet;
+import java.util.Set;
+
 public class Triangle {
 
     private double side1;

triangle/src/example/java/TriangleException.java

+../../main/java/TriangleException.java

triangle/src/example/java/TriangleKind.java

+../../main/java/TriangleKind.java