Provide asn_n_members=1 when opening dataset for Step to call get_crds_parameters

[braingram]

This PR improves the consistency of the parameters used for _datamodels_open by providing asn_n_members=1 for Step (to match what is done in Pipeline).

Fixes spacetelescope/jwst#8262

Romancal regtests all pass: https://plwishmaster.stsci.edu:8081/job/RT/job/Roman-Developers-Pull-Requests/608/
JWST regtests show 4 unrelated errors: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1226/
(the same errors are seen in a run without this PR using the current JWST default branch: ttps://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1227/

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 59.33%. Comparing base (aea1617) to head (8c8bdd2).
Report is 1 commits behind head on main.

❗ Current head 8c8bdd2 differs from pull request most recent head 66012fb. Consider uploading reports for the commit 66012fb to get more accurate results

Files	Patch %	Lines
src/stpipe/step.py	0.00%	3 Missing ⚠️
src/stpipe/pipeline.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
+ Coverage   59.15%   59.33%   +0.18%     
==========================================
  Files          24       24              
  Lines        1611     1606       -5     
==========================================
  Hits          953      953              
+ Misses        658      653       -5     

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[nden]

This looks good.
There's a bug in roman_datamodels - it deletes asn_n_members.
In addition _datamodels_open does not work with ModelContainer and ModelContainer does not accept asn_n_memers. A ticket describing the issues is https://jira.stsci.edu/browse/RCAL-777