add Euclid instrument

[fjaviersanchez]

I'm opening this issue to keep track of the work to implement the Euclid instrument/filters in WLD. The main issue is going to be getting the correct magnitudes for the OneSqDeg.fits catalog.

[fjaviersanchez]

I have a version of the OneSqDeg.fits including VIS magnitudes using the latest version of Euclid VIS throughputs (see desihub/speclite#47 for more details about the filters). Should I post it somewhere (Google Drive or git lfs?)?