JP-3521: Fix jump multiprocessing error

[mwregan2]

Resolves JP-3521

Closes spacetelescope/jwst#8221

This PR addresses the different results return from the Jump step when switching from single core processing to multiprocessing. The main problem was caused by calculating the number of rows per slice using floating point division. This lead to some rows not getting processed.
In addition, the 4 neighbor flagging in multiprocessing was adding the jump bit to pixels that were already flagged as do_not_use or saturated.

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (30c41af) 84.31% compared to head (a061bdd) 85.94%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #239      +/-   ##
==========================================
+ Coverage   84.31%   85.94%   +1.62%     
==========================================
  Files          35       35              
  Lines        6452     6523      +71     
==========================================
+ Hits         5440     5606     +166     
+ Misses       1012      917      -95     

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[hbushouse]

Everything seems to work now.