MAINT: Clean up set_algo_instance usage in TradingAlgorithm.

TradingAlgorithm always uses set_algo_instance in pairs of
set_algo_instance(self) and set_algo_instance(None).  Refactoring this to use a
context manager.

zipline/utils/api_support.py

@@ -28,6 +28,29 @@ def set_algo_instance(algo):
     context.algorithm = algo
 
 
+class ZiplineAPI(object):
+    """
+    Context manager for making an algorithm instance available to zipline API
+    functions within a scoped block.
+    """
+
+    def __init__(self, algo_instance):
+        self.algo_instance = algo_instance
+
+    def __enter__(self):
+        """
+        Set the given algo instance, storing any previously-existing instance.
+        """
+        self.old_algo_instance = get_algo_instance()
+        set_algo_instance(self.algo_instance)
+
+    def __exit__(self, _type, _value, _tb):
+        """
+        Restore the algo instance stored in __enter__.
+        """
+        set_algo_instance(self.old_algo_instance)
+
+
 def api_method(f):
     # Decorator that adds the decorated class method as a callable
     # function (wrapped) to zipline.api