JP-3547: Fix MIRI array sizing for multiple inputs and pixel ratio != 1

[melanieclarke]

Resolves JP-3547

Fixes an issue discovered during testing for #8596.

For MIRI, when there are multiple input files and pixel_scale_ratio is set to something other than 1.0, the output array size was being computed incorrectly: the pixel_scale_ratio was applied twice. This PR fixes that issue and adds a unit test for the multiple input case.

Checklist for PR authors (skip items if you don't have permissions or they are not applicable)

• [x] added entry in CHANGES.rst within the relevant release section
• [x] updated or added relevant tests
• [ ] ~updated relevant documentation~
• [x] added relevant milestone
• [x] added relevant label(s)
• [x] ran regression tests, post a link to the Jenkins job below. How to run regression tests on a PR
• [x] All comments are resolved
• [x] Make sure the JIRA ticket is resolved properly

[melanieclarke]

Regression tests here: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1677

[codecov[bot]]

Codecov Report

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

Project coverage is 60.55%. Comparing base (e90cafa) to head (f78361d). Report is 7 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8727 +/- ## ======================================= Coverage 60.55% 60.55% ======================================= Files 372 372 Lines 38371 38371 ======================================= Hits 23235 23235 Misses 15136 15136 ```

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[melanieclarke]

Regtest failures all look unrelated - there are no changes to any spectral resampling products.

[drlaw1558]

LG2M: Fixes the issue with LRS slit data, and results look as expected in a test case of spec3-combined dithered data.

[melanieclarke]

@tapastro - can you please review for the maintainers?