timezone aware

[AlanKuurstra]

addresses #777

Adds capability to handle datetime stripping for all datetimes allowed in the dicom and bids standards.

[yarikoptic]

dang, I have managed to miss this PR and CI managed to not run test for some reason!
I will attend to it within a few days. If you could rebase or otherwise refresh it to re-trigger the CI, would be appreciated.

[AlanKuurstra]

I rebased, CI is awaiting approval from a maintainer.

[yarikoptic]

Thanks again for the PR! Let's get "zone aware!" ;-)
See individual comments for some minor tune ups requested

[codecov]

Codecov Report

Attention: Patch coverage is 97.87234% with 2 lines in your changes missing coverage. Please review.

Project coverage is 82.39%. Comparing base (383525d) to head (d47bc9c).

Files with missing lines	Patch %	Lines
heudiconv/utils.py	96.15%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #780      +/-   ##
==========================================
+ Coverage   82.13%   82.39%   +0.25%     
==========================================
  Files          42       42              
  Lines        4226     4299      +73     
==========================================
+ Hits         3471     3542      +71     
- Misses        755      757       +2     

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