Add order argument to ts.nodes()

Fixes tskit-dev#2370

python/tskit/trees.py

@@ -3802,8 +3802,11 @@ class SimpleContainerSequence:
     python sequence, allowing forward and reverse iteration.
     """
 
-    def __init__(self, getter, length):
-        self.getter = getter
+    def __init__(self, getter, length, order=None):
+        if order is None:
+            self.getter = getter
+        else:
+            self.getter = lambda index: getter(order[index])
         self.length = length
 
     def __len__(self):
@@ -4463,15 +4466,29 @@ def individuals(self):
         """
         return SimpleContainerSequence(self.individual, self.num_individuals)
 
-    def nodes(self):
+    def nodes(self, *, order=None):
         """
         Returns an iterable sequence of all the :ref:`nodes <sec_node_table_definition>`
         in this tree sequence.
 
+        :param str order: The order in which the nodes should be returned: must be
+            one of "id" (Default), "timeasc" (ascending order of time, then by ascending
+            node id), or "timedesc" (descending order of time, then by ascending
+            node id).
         :return: An iterable sequence of all nodes.
         :rtype: Sequence(:class:`.Node`)
         """
-        return SimpleContainerSequence(self.node, self.num_nodes)
+        order = "id" if order is None else order
+        if order not in ["id", "timedesc", "timeasc"]:
+            raise ValueError('order must be "id", "timedesc", or "timeasc"')
+        if order == "id":
+            odr = None
+        elif order == "timeasc":
+            odr = np.lexsort((np.arange(self.num_nodes), self.tables.nodes.time))
+        elif order == "timedesc":
+            odr = np.lexsort((np.arange(self.num_nodes), -self.tables.nodes.time))
+
+        return SimpleContainerSequence(self.node, self.num_nodes, order=odr)
 
     def edges(self):
         """