added slice helper method for strings and modulo for integers

VectoCore/Utils/IntExtensionMethods.cs

-using System;
-using System.Diagnostics.Contracts;
-
 namespace TUGraz.VectoCore.Utils
 {
 	public static class IntExtensionMethods
@@ -34,5 +31,16 @@ namespace TUGraz.VectoCore.Utils
 		{
 			return SIBase<T>.Create(d);
 		}
+
+        /// <summary>
+        /// Modulo functions which also works on negative Numbers (not like the built-in %-operator which just returns the remainder).
+        /// </summary>
+        /// <param name="a"></param>
+        /// <param name="b"></param>
+        /// <returns></returns>
+        public static int Mod(this int a, int b)
+        {
+            return (a %= b) < 0 ? a + b : a;
+        }
 	}
 }
\ No newline at end of file

VectoCore/Utils/StringExtensionMethods.cs

+using System;
 using System.Globalization;
 
 namespace TUGraz.VectoCore.Utils
@@ -8,5 +9,14 @@ namespace TUGraz.VectoCore.Utils
 		{
 			return double.Parse(self, CultureInfo.InvariantCulture);
 		}
+
+        public static string Slice(this string s, int from = 0, int to = int.MaxValue)
+        {
+            from = Math.Min(Math.Max(from, -s.Length), s.Length);
+            from = from < 0 ? from + s.Length : from;
+            to = Math.Min(Math.Max(to, -s.Length), s.Length);
+            to = to < 0 ? to + s.Length : to;
+            return s.Substring(from, Math.Max(to - from, 0));
+        }
 	}
 }
\ No newline at end of file