feat(Utils): add a .IndexOutputs function to index OutputsModel objects

- to be replaced by '[.OutputsModel' in airGR 2.0
Refs # 122

R/Utils.R

@@ -218,16 +218,17 @@
   res
 }
 
+.IndexOutputsModel <- function(x, i) {
 # '[.OutputsModel' <- function(x, i) {
-#   if (!inherits(x, "OutputsModel")) {
-#     stop("'x' must be of class 'OutputsModel'")
-#   }
-#   if (is.factor(i)) {
-#     i <- as.character(i)
-#   }
-#   if (is.numeric(i)) {
-#     .ExtractOutputsModel(x, i)
-#   } else {
-#     NextMethod()
-#   }
-# }
+  if (!inherits(x, "OutputsModel")) {
+    stop("'x' must be of class 'OutputsModel'")
+  }
+  if (is.factor(i)) {
+    i <- as.character(i)
+  }
+  if (is.numeric(i)) {
+    .ExtractOutputsModel(x, i)
+  } else {
+    NextMethod()
+  }
+}