add multi-argument select for C#

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/api/dotnet/ArrayExpr.cs

@@ -44,5 +44,10 @@ namespace Microsoft.Z3
             get { return Context.MkSelect(this, index); }
         }
 
+        public Expr this[IEnumerable<Expr> index] 
+        {
+            get { return Context.MkSelect(this, index.ToArray()); }
+        }
+
     }
 }

src/api/dotnet/Context.cs

@@ -2079,6 +2079,7 @@ namespace Microsoft.Z3
             return Expr.Create(this, Native.Z3_mk_select_n(nCtx, a.NativeObject, AST.ArrayLength(args), AST.ArrayToNative(args)));
         }
 
+
         /// <summary>
         /// Array update.
         /// </summary>