Add linters to deltametrics: flake8

[mcflugen]

Adds the flake8 linter to deltametrics and fixes some issues. For the most part, this is just removing unused imports and variables.

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 89.35%. Comparing base (a429894) to head (e445808).
Report is 12 commits behind head on develop.

Files with missing lines	Patch %	Lines
deltametrics/plan.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #186      +/-   ##
===========================================
+ Coverage    89.33%   89.35%   +0.01%     
===========================================
  Files            9        9              
  Lines         3620     3617       -3     
===========================================
- Hits          3234     3232       -2     
+ Misses         386      385       -1     

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