Zeropoint and zeropoint_airmass references

[mpaillassa]

This PR aims to update the zeropoint references (especially the speclite ones) and to add the zeropoint_airmass references that were missing.

(I did not update Euclid as it is done in its corresponding PR #49)

[aboucaud]

Currently the zeropoint_airmass is an optional parameter.
I think we should ignore it instead of writing zeropoint_airmass=0.0 for space surveys so we can test for instance :

for surveys in available_surveys:
    if survey.zeropoint_airmass is not None:
        ...

What do you think ?

[ismael-mendoza]

I think we should ignore it instead of writing zeropoint_airmass=0.0 for space surveys so we can test for instance :

I'm usually in favor of avoiding if statements and branching in code if possible. Could you tell us a little more of why testing this with an if statement might be useful even if setting zeropoint_airmass = 0.0 for space telescopes makes sense?

[aboucaud]

Thanks for your comment @ismael-mendoza.
After some thoughts I agree with you.
In that specific case, putting 0.0 makes sense and is not just a "fill value".

So since we have values for all surveys I'll make this parameter not optional in an upcoming PR and merge this one.