Adds option to specify config location using env variable

[aturner-epcc]

Adds the option to set CATS_CONFIG_FILE environment variable to specify config file location. Config file location precedence becomes:

1. Command line option
2. Location specified in CATS_CONFIG_FILE
3. ./config.yaml

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 33.33333% with 6 lines in your changes missing coverage. Please review.

Project coverage is 88.33%. Comparing base (31cfede) to head (eeb961c). Report is 22 commits behind head on main.

Files with missing lines	Patch %	Lines
cats/configure.py	33.33%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   89.23%   88.33%   -0.91%     
==========================================
  Files          14       14              
  Lines         548      557       +9     
==========================================
+ Hits          489      492       +3     
- Misses         59       65       +6     

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[andreww]

I've rebased this and added a test. See PR #121 which contains this patch