Clarify spherex 0 vs 1 indexing

[troyraen]

• 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 #288 (comment)

Needs to be clarified in the PSF notebook, and possibly others.

[afaisst]

While the zone pixel coordinate centers are 0-based, their names in the header (XCTR_1, etc) are 1-based.
This has been clarified in the text. Furthermore, the formula for the calculation of the distance between pixel and PSF zone centers has been updated to account for 0-based indexing.

[afaisst]

PR: #293