Return ROS time values

[liambenson]

Fixes: DATA-627

Description of Changes

This switches the python interface to providing timestamps as ROS times, similar to how the actual rospy interface works.

Test Plan

Unit tests.

[kashish-jain]

I'm wondering if we need to support getting time as int or float? Like, why can't we just return embag.RosTime and the client can call to_sec or to_nsec to get long/int or float. This way we won't have to pass around that ros_time_py_type. Does rospy support just getting time/duration as primitive? May be this is for backward compatibility?

[liambenson]

I'm wondering if we need to support getting time as int or float? Like, why can't we just return embag.RosTime and the client can call to_sec or to_nsec to get long/int or float. This way we won't have to pass around that ros_time_py_type. Does rospy support just getting time/duration as primitive? May be this is for backward compatibility?

@kashish-jain Its mostly just about providing nice functionality for clients so that they don't have to worry about writing the conversions/serializations themselves and dealing with performance hits from deeply nested structures. IMO passing the arg around isn't too big of a deal.

rospy doesn't provide any to_dict functionality, so theres no precedence there.

[kashish-jain]

🚢