Default parallel to False

[iguinn]

See issue #592. This is meant to be a temporary fix.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 48.87%. Comparing base (68cebf5) to head (ddb25b7).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pygama/flow/data_loader.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #593   +/-   ##
=======================================
  Coverage   48.87%   48.87%           
=======================================
  Files          59       59           
  Lines        7863     7863           
=======================================
  Hits         3843     3843           
  Misses       4020     4020           

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