RCAL-800 Add initialization for the flux step meta

[stscieisenhamer]

Resolves RCAL-800

Related PRs:

• rad#395
• romancal#1160

This PR adds the initialization for the cal_step.flux meta in the maker utilities.

Checklist

• [x] Added entry in CHANGES.rst under the corresponding subsection
• [x] updated relevant tests
• [x] updated relevant documentation
• [ ] Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.34%. Comparing base (087a60d) to head (e175ded). Report is 8 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #332 +/- ## ========================================== - Coverage 97.56% 97.34% -0.22% ========================================== Files 30 32 +2 Lines 2788 2980 +192 ========================================== + Hits 2720 2901 +181 - Misses 68 79 +11 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.