Add Gaia spectra and SED errors + small debugs

[jeremyneveu]

• Use gaiaspec library from @corentinravoux
• add SED uncertainty in forward modelling
• debugging (in particular "noprior" method and "least_squares" method)

[jeremyneveu]

@corentinravoux are you happy with those changes for now?

[coveralls]

Pull Request Test Coverage Report for Build 11854660698

Details

• 82 of 164 (50.0%) changed or added relevant lines in 6 files are covered.
• 20 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.4%) to 88.841%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spectractor/fit/fit_spectrogram.py	5	14	35.71%
spectractor/fit/fit_spectrum.py	4	13	30.77%
spectractor/extractor/targets.py	59	123	47.97%

Files with Coverage Reduction	New Missed Lines	%
spectractor/extractor/targets.py	1	67.1%
spectractor/extractor/background.py	1	96.88%
spectractor/extractor/spectrum.py	6	92.05%
spectractor/extractor/images.py	12	89.79%

Totals
Change from base Build 11841890043:	-0.4%
Covered Lines:	7635
Relevant Lines:	8594

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💛 - Coveralls

[corentinravoux]

I created a first version of gaiaspec (https://github.com/corentinravoux/gaiaspec/releases/tag/v1.0.0) which is the one that I am using with Spectractor right now

[corentinravoux]

The current gaiaspec version worked on usdf with the commit "add sed error". From the changes I am seeing on the "small debugs" commit, I do not think it can cause a failure, at least on the gaiaspec side