Improve some headers and message display

[olivierilbert]

Minor improvements for the display:

• Offsets from auto adapt written twice (once in prep_data, once in zphota.py). Remove one of them.
• More informative error message when a file is missing
• Change the header of the ascii file for the predicted magnitude library

[olivierilbert]

I am going to close issue #261 by Mara and check the few places were we display information using the c++ convention starting indexing at 0, while it would be more clear to start at 1.

[olivierilbert]

Ready to merge in the main, despite this failure in "Unit test and code coverage / build (3.9) (pull_request)"