Implement SeriesIntersection.inverse()

Implement SeriesIntersection.inverse() to return the inverse of the
current intersection type

lib/xmet/series.py

@@ -17,6 +17,14 @@ class SeriesIntersection(enum.Enum):
         elif a < b:
             return SeriesIntersection.LESSER
 
+    def inverse(self):
+        if self is SeriesIntersection.GREATER:
+            return SeriesIntersection.LESSER
+        elif self is SeriesIntersection.LESSER:
+            return SeriesIntersection.GREATER
+
+        return self
+
 class Series(dict):
     def __init__(self):
         super().__init__()