ensuring resolution does not contain ppi

[mwarin]

Fix for https://hathitrust.atlassian.net/browse/GS-13230

... in which some of bc's IA items were failing because they fall back on an invalid resolution value from the $volume.
Single line fix that removes ppi from resolution values.

This should fail on the main branch:

$ git checkout main
$ docker compose run --rm test perl bin/ingest_ia_dev.pl realceduladesuma00spai_0 test ark:/13960/s25zng884bb
[...]
Error setting new tag Resolution => 400ppi: Not a floating point number for MIE-Image:Resolution

... but should succeed on this PR branch:

$ git checkout GS-13230-ppi-resolution-fix
$ docker compose run --rm test perl bin/ingest_ia_dev.pl realceduladesuma00spai_0 test ark:/13960/s25zng884bb

[mwarin]

@aelkiss I would like a chance to run this through with you once or twice. I think there are dueling red herrings:

• can't run it as namespace:test because it's going to look for things in zephir and fail
• can't run it as namespace:bc in dev because I don't know how to make real namespaces work in dev
• once i manage to get it in zephir, the arkid on https://zephir.cdlib.org/api/item/bc.ark:/13960/s25zng884bb does not match the arkid realceduladesuma00spai_0

[aelkiss]

After testing with the correct IA ID matching this arkid, everything looks good. We added ETT-340 as a potential future step to make testing this kind of thing easier.