Update jump.py to limit the number of slices

[mwregan2]

For multi-processing, make sure that the number of slices is not larger than the number of rows. Fixes failures seen in unit tests when the number of cores is "all".

Checklist

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

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.02 :warning:

Comparison is base (a7351de) 74.47% compared to head (669a06f) 74.46%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #176 +/- ## ========================================== - Coverage 74.47% 74.46% -0.02% ========================================== Files 29 29 Lines 5936 5937 +1 ========================================== Hits 4421 4421 - Misses 1515 1516 +1 ``` | [Impacted Files](https://app.codecov.io/gh/spacetelescope/stcal/pull/176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | |---|---|---| | [src/stcal/jump/jump.py](https://app.codecov.io/gh/spacetelescope/stcal/pull/176?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope#diff-c3JjL3N0Y2FsL2p1bXAvanVtcC5weQ==) | `63.79% <0.00%> (-0.23%)` | :arrow_down: |

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