Fix 1080

[CBroz1]

Description

#1053 made an edit to _compute_metric control flow anticipating an early return, but instead allowed isi_violation to be recomputed for each unit. This is present in the most recent release, but will result in failed saving, as shown in #1080, because the embedded dictionary will not be able to save to json

Checklist:

• No. This PR should be accompanied by a release: (yes/no/unsure)
• N/a. If release, I have updated the CITATION.cff
• No. This PR makes edits to table definitions: (yes/no)
• N/a. If table edits, I have included an alter snippet for release notes.
• No. If this PR makes changes to position, I ran the relevant tests locally.
• I have updated the CHANGELOG.md with PR number and description.
• N/a. I have added/edited docs/notebooks to reflect the changes