Feature/accept datetime on retrieve

[AlcibiadesCleinias]

resolve #53

But: I did not update docs. Not sure how to do it right

PS.
It is better to review and merge after #572 coz the pr consists of some features of 572 pr

[Hugovdberg]

I would like you to make a few changes to the new time module to keep Python 2.7 support for now. I like that you put this in a separate module, I think that the PISeries.timestamp_to_index function could be moved in there as well.

By the way, I already made a new branch remove_python2_support to which should be Python 3 only, and include type hints everywhere, feel free to post contributions to that branch if you want to help strip out the Python 2 compatibility. Once PIconnect 0.9 is released I want to merge that branch into develop.

[AlcibiadesCleinias]

I think that the PISeries.timestamp_to_index function could be moved in there as well.

Think the same: done

By the way, I already made a new branch remove_python2_support to which should be Python 3 only, and include type hints everywhere, feel free to post contributions to that branch if you want to help strip out the Python 2 compatibility

Seems like a good idea. I will checkout

[AlcibiadesCleinias]

oh, ought to mention: no docs about new module generated...

[Hugovdberg]

I think not documenting the new module is fine for now, as it is meant to be used internally only. I'm working on improving the documentation structure so it becomes more clear where to add the new documentation.