[algorithm] Add write method to runner

This method will be called on the loop when
an output is written by the loop user algorithm.

beat/backend/python/algorithm.py

@@ -325,6 +325,24 @@ class Runner(object):
 
         return answer
 
+    def write(self, outputs):
+        """Write to the outputs"""
+
+        exc = self.exc or RuntimeError
+
+        if self.algorithm.type != Algorithm.LOOP:
+            raise exc("Wrong algorithm type: %s" % self.algorithm.type)
+
+        # setup() must have run
+        if not self.ready:
+            raise exc("Algorithm '%s' is not yet setup" % self.name)
+
+        # prepare() must have run
+        if not self.prepared:
+            raise exc("Algorithm '%s' is not yet prepared" % self.name)
+
+        return loader.run(self.obj, "write", self.exc, outputs)
+
     def __getattr__(self, key):
         """Returns an attribute of the algorithm - only called at last resort
         """