Implement Series.append() to add onto a Series

lib/xmet/series.py

@@ -29,6 +29,10 @@ class Series(dict):
     def __init__(self):
         super().__init__()
 
+    def append(self, series: Self):
+        for key in series:
+            self[key] = series[key]
+
     def neighbors(self, series: Self):
         return nearest(sorted(self.keys(),   reverse=True),
                        sorted(series.keys(), reverse=True))