Refactor generate page set script to (1) support nonconsecutive works and (2) simplify page selection logic

[laurejt]

• Support nonconsecutive PPA works

• Simplify page selection

[rlskoeser]

To improve handling of PPA excerpt page ranges, I recommend we use intspan. In ppa-django we use the intspan package to parse page ranges, and anything in the data exports is guaranteed parsable by intspan.

Here's where we use it in the ppa-django DigitizedWork model: https://github.com/Princeton-CDH/ppa-django/blob/main/ppa/archive/models.py#L1047-L1053

It returns an object that can be treated as an iterable with all page numbers included in the span.