Add linters to deltametrics: flake8-bugbear

[mcflugen]

Adds the flake8-bugbear linter to deltametrics and fixes some issues.

Maybe the two biggest changes here are:

1. Set stacklevel=2 for warnings. The default of 1 produces warnings that can be difficult to understand.
2. Use only immutable objects as keyword defaults. If you use a mutable object as a default and then change that object's value within the function, the default value for subsequent calls of the function will also change.

[codecov]

Codecov Report

Attention: Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.40%. Comparing base (8359596) to head (30156f4).
Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
deltametrics/utils.py	0.00%	2 Missing ⚠️
deltametrics/plan.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #188      +/-   ##
===========================================
+ Coverage    89.35%   89.40%   +0.05%     
===========================================
  Files            9        9              
  Lines         3617     3616       -1     
===========================================
+ Hits          3232     3233       +1     
+ Misses         385      383       -2     

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