Add start_with_unprocessed option to auto-pipeline

[nicHoch]

for a transition from batch to daily processing and also for a restart after some error time / system down. We need an option for the automated pipeline to not just process new incoming files but also start with a search for unprocessed files and process them first.

[codecov-commenter]

Codecov Report

Base: 80.51% // Head: 80.85% // Increases project coverage by +0.33% 🎉

Coverage data is based on head (1955d57) compared to base (4f31099).
Patch coverage: 51.85% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #278      +/-   ##
==========================================
+ Coverage   80.51%   80.85%   +0.33%     
==========================================
  Files          60       62       +2     
  Lines        5697     5927     +230     
==========================================
+ Hits         4587     4792     +205     
- Misses       1110     1135      +25     

Impacted Files	Coverage Δ
stixcore/util/scripts/end2end_testing.py	24.56% <ø> (ø)
stixcore/processing/pipeline.py	71.23% <22.22%> (-4.51%)	⬇️
stixcore/products/level0/scienceL0.py	84.68% <57.62%> (-1.95%)	⬇️
stixcore/time/datetime.py	90.68% <100.00%> (ø)
stixcore/io/fits/tests/test_processors.py	100.00% <0.00%> (ø)
stixcore/products/levelb/tests/test_levelb.py	100.00% <0.00%> (ø)

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[samaloney]

Looks ok but really hard to be sure 🤷‍♂️