Refactoring of the API to improve error handling

What's Changed

• Add Rubin and Euclid filter central wv by @aboucaud in #13
• Fix value for Euclid PSF FWHM by @Hbretonniere in #9
• Add contribution guidelines by @aboucaud in #14
• Merge survey and instrument concepts by @aboucaud in #17
• Small adjustments by @aboucaud in #18
• Change API for accessing the surveys by @aboucaud in #19
• Fix readme by @aboucaud in #20

New Contributors

• @Hbretonniere made their first contribution in #9

Full Changelog: 0.0.4...0.1.0