Resolve "Fix large grid display"

Closes #110 (closed)

game-of-life/src/main/java/fr/ippon/life/Grid.java

@@ -8,10 +8,12 @@ import java.util.stream.Stream;
 
 public class Grid {
 
+  private final Set<Cell> aliveCells;
   private final Set<Cell> deadCells;
   private final Box border;
 
   Grid(Set<Cell> aliveCells) {
+    this.aliveCells = aliveCells;
     this.deadCells = buildDeadCells(aliveCells);
     border = new Box(aliveCells);
   }
@@ -25,6 +27,10 @@ public class Grid {
       .collect(Collectors.toUnmodifiableSet());
   }
 
+  public boolean isAlive(Cell cell) {
+    return aliveCells.contains(cell);
+  }
+
   public Collection<Cell> deadCells() {
     return deadCells;
   }