Add a simpler aproximate time sync policy: ApproximateEpsilonTime

[ivanpauno]

This adds another approximate time policy. The difference is that it doesn't use a relatively complex algorithm to check if the topics are in sync.

It works like ExactTime, but accepts timestamps being within an "epsilon" tolerance.

Todo:

• [x] Add test cases.

(I have done some manual testing using this in stereo image proc, it seems to work fine)

[ivanpauno]

• Linux
• Linux-aarch64
• Windows

[ivanpauno]

@gbiggs could you take another look?

[ivanpauno]

Thanks for the reviews @jacobperron @gbiggs !!!

[ZhenshengLee]

It works like ExactTime, but accepts timestamps being within an "epsilon" tolerance.

According to our test, the epsilon version causes less latency to get a good enough sync results in a 8-cameras image pipeline.

Thanks!