Convert index data time to rostime on bag write

[klane]

The reader converts index data time from rostime here. However, the writer does not convert it to rostime.

[defunctzombie]

@klane For clarity on the issue - what problem does this cause because the conversion is not happing on the writer?

[klane]

@defunctzombie We have some downstream Python code that reads bags using the rosbags library. It assumes index data times are in rostime format. As is, messages come in unordered with incorrect times. Converting times in the writer resolves this for us.

[james-rms]

@klane thanks for finding this! I've created a PR here which includes this change, a test and some doc changes to clarify, closing this one in favor of that.