Use concurrent.futures for parallel exposures.

[wtgee]

Better parallel exposures for the take-pic cli command. This has been used on unit 007 for a while and needs to be merged.

[codecov]

Codecov Report

Attention: Patch coverage is 20.00000% with 20 lines in your changes missing coverage. Please review.

Project coverage is 70.27%. Comparing base (f665383) to head (5d09823).
Report is 6 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/panoptes/pocs/utils/cli/camera.py	20.00%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1293      +/-   ##
===========================================
- Coverage    70.36%   70.27%   -0.10%     
===========================================
  Files          101      101              
  Lines         8568     8582      +14     
  Branches       719      722       +3     
===========================================
+ Hits          6029     6031       +2     
- Misses        2389     2400      +11     
- Partials       150      151       +1     

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