Handle different start and end times of components

• connect and _connect get an argument start_time that is to be passed to try_connect instead of self.time
• Connect helper pushes data twice: once for start_time and once for info.time (if it is not equal to start_time)
• Starting time can be passed to component's run(), and is auto-determined if not provided

Basically, only the changes in interfaces.py, schedule.py, component.py and connect_helper.py are of interest. The rest is fixes for the breaking change to the interface.

Fixes #4 (closed)

src/finam/sdk/component.py

 """
 Abstract base implementations for components with and without time step.
 """
-import datetime
 import collections
+import datetime
 import logging
 from abc import ABC
 from datetime import datetime
@@ -440,7 +440,8 @@ class TimeComponent(ITimeComponent, Component, ABC):
         if self._time is None and self.status in (
             ComponentStatus.CREATED or ComponentStatus.INITIALIZED
         ):
-            """A time of None is ok before the connect phase"""
+            return None
+
         if not isinstance(self._time, datetime):
             with ErrorLogger(self.logger):
                 raise ValueError("Time must be of type datetime")