MAINT: Standardize reader get value methods.

The daily/session bar reader's `spot_price` took the same parameters and
returned the same kind of output as the minute bar reader's `get_value`.

Standardize on one method to make a common interface, which may be
formally factored out in a later patch; to help enable writing reader
implementations or mixins which can be agnostic to the bar frequency.

zipline/data/session_bars.py

@@ -43,7 +43,7 @@ class SessionBarReader(with_metaclass(ABCMeta)):
         pass
 
     @abstractmethod
-    def spot_price(self, sid, session, colname):
+    def get_value(self, sid, session, colname):
         """
         Retrieve the value at the given coordinates.