Handle duplicate connection outputs in TOUGH3 and zombie subprocesses

[keurfonluu]

• Fixed: some earlier versions of TOUGH3 have duplicate connection outputs when running in parallel. toughio.read_output now look for duplicate connections and merge them.
• Fixed: when manually stopped within a Python interpreter, toughio.run does not terminate the child process, resulting in a defunct zombie process. toughio.run now catches errors while running and send a termination signal to the parent and its child processes.
• Fixed: allow non string arguments in docker_args.

Reminders:

• Run invoke format to make sure the code follows the style guide,
• Add tests for new features or tests that would have caught the bug that you're fixing,
• Write detailed docstrings for all functions, classes and/or methods,
• If adding new functionality, unit test it and add it to the documentation.

[codecov]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (6844260) 90.25% compared to head (00363f6) 90.21%.
Report is 13 commits behind head on master.

Files	Patch %	Lines
toughio/_run.py	40.90%	13 Missing ⚠️
toughio/_io/output/_common.py	66.66%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   90.25%   90.21%   -0.05%     
==========================================
  Files          97       97              
  Lines        6260     6284      +24     
==========================================
+ Hits         5650     5669      +19     
- Misses        610      615       +5     

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