Simplify rosbag2_py message deserialization APIs

[mabelzhang]

Description

As a followup to #308, it would be nice to wrap around rosidl_runtime_py.utilities.get_message and rclpy.serialization.deserialize_message currently needed in the Python API, so that the Python function calls look nicer.

As a further step, it would be nice to have function calls similar to ROS 1 rosbag Python API http://wiki.ros.org/rosbag/Code%20API#Python_API

Related Issues

PR #308

Completion Criteria

Simplified Python API.
If appropriate, allow passing in a list of topics in Python.

Implementation Notes / Suggestions

One difference from ROS 1 is that where ROS 1 passes a list of topics into bag.read_messages, in rosbag2, the filter is at the storage level (rosbag2_storage/storage_filter.hpp), and the filter should be set via reader.set_filter() before reading the messages (see #302). This Python API is already implemented in the reader test of #308.

Testing Notes / Suggestions

Change rosbag2_py/test/test_sequential_reader and test_sequential_writer to use the new syntax instead of get_message and deserialize_message.