Outputs cache, clear and interpolate data

Preparation to solve build new scheduling algorithm. See #78 (closed).

Use the ping mechanism to let outputs know their targets.

Outputs record pull time per target to determine data to store/clear. Store data, and clear after pull what can't be pulled anymore with strictly increasing pull time per target.

Outputs do nearest interpolation on pull and throw an arror when out of the data range.

Fixes #76 (closed)

src/finam/sdk/output.py

@@ -211,7 +211,8 @@ class Output(IOutput, Loggable):
 
             t_prev, data_prev = self.data[i - 1]
             diff = t - t_prev
-            t_half = t_prev + diff
+            t_half = t_prev + diff / 2
+
             if time < t_half:
                 return data_prev