JP-3930: Make sure input is not modified inside Steps

[melanieclarke]

Resolves JP-3930

Adding smoke tests for all steps for as many input cases as possible and fixing any issues with input modification revealed by the tests.

Some steps addressed separately:
background: #9688
coron: #9693
outlier_detection: #9709
skymatch, tweakreg, assign_mtwcs: #9723
tso_photometry: #9579 and/or #9627

Tasks

• If you have a specific reviewer in mind, tag them.
• add a build milestone, i.e. Build 12.0 (use the latest build if not sure)
• Does this PR change user-facing code / API? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see changelog readme for instructions)
  • update or add relevant tests
  • update relevant docstrings and / or docs/ page
  • start a regression test and include a link to the running job (click here for instructions)
    • Do truth files need to be updated ("okified")?
      • after the reviewer has approved these changes, run okify_regtests to update the truth files
• if a JIRA ticket exists, make sure it is resolved properly

[tapastro]

Looks good to me! I don't see any lingering questions. The last RT run was clean - I assume we are not expecting failures? We can get this in and start a new run to be safe.

[melanieclarke]

Looks good to me! I don't see any lingering questions. The last RT run was clean - I assume we are not expecting failures? We can get this in and start a new run to be safe.

No expected failures! I'll go ahead and merge and start a new run.