group and subgroup adding/editing, services added

TimetableDesigner.Core/Group.cs

@@ -6,15 +6,27 @@ using System.Threading.Tasks;
 
 namespace TimetableDesigner.Core
 {
-    public class Group
+    public class Group : IGroup
     {
         #region PROPERTIES
 
         public string Name { get; set; }
         public string Description { get; set; }
-        public Subgroup MainSubgroup { get; set; }
         public ICollection<Subgroup> AssignedSubgroups { get; set; }
 
         #endregion
+
+
+
+        #region CONSTRUCTORS
+
+        public Group()
+        {
+            Name = string.Empty;
+            Description = string.Empty;
+            AssignedSubgroups = new HashSet<Subgroup>();
+        }
+
+        #endregion
     }
 }