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/testing/core.py

@@ -959,7 +959,7 @@ def subtest(iterator, *_names):
 
 
 class MockDailyBarReader(object):
-    def spot_price(self, col, sid, dt):
+    def get_value(self, col, sid, dt):
         return 100