Add missing _where_index.py

Author: smathot

datamatrix/_datamatrix/_where_index.py

@@ -0,0 +1,19 @@
+class WhereIndex(list):
+    """Primarily serves as a list-like object that is returned with where-like
+    operations:
+    
+        where_index = dm[dm.col == 1]
+
+    However, for compariblity with dataframe, this list-like object should also
+    be able to provide access to columns, like so:
+    
+        dm[dm.col == 1]['col'] = 2
+    """    
+    def __init__(self, dm, values):
+        self._dm = dm
+        super().__init__(values)
+    
+    def __getitem__(self, key):
+        if not isinstance(key, int):
+            return self._dm[key]
+        return key