Implement Series.nearest()

Implement Series.nearest() to allow finding value by nearest key in a
Series object

lib/xmet/series.py

@@ -50,3 +50,8 @@ class Series(dict):
 
             if sign is intersection:
                 return v1, pair[0]
+
+    def nearest(self, key):
+        k = sorted(self.keys(), key=lambda k: abs(k-key))[0]
+
+        return self[k]