Clarify spherex 0 vs 1 indexing

> - Another bug is that the PSFZones are 0-based but should be 1-based.  We also need to make this consistent.

_Originally posted by @christinanelson in https://github.com/Caltech-IPAC/irsa-tutorials/issues/288#issuecomment-4056952085_

Needs to be clarified in the PSF notebook, and possibly others.