Generalize IIGlue reader to read multiple iiglue analysis output files

We originally discussed making these changes on the [`sentinel-checks`](https://github.com/AlisaMaas/CArrayIntrospection/tree/sentinel-checks) branch. However, nothing here is really associated with sentinel checks _per se_. So I thought it made more sense for this to be a feature branch of its own. @AlisaMaas, I hope that doesn't create extra merging headaches for you. Can you just merge this over to your [`sentinel-checks`](https://github.com/AlisaMaas/CArrayIntrospection/tree/sentinel-checks) branch whenever you are ready for it?

Don't be fooled by the apparent size of the changes to `IIGlueReader.cc`. As noted in e45c35c74537d9aad85ffde844e45c877a24804a, adding a new loop around existing code caused lots of indentation changes. The non-white-space changes are actually quite small. Unfortunately, GitHub gets confused about the correspondences between old and new lines, making the diff look bigger than it really should be.