support custom coadds, update line wavelengths; major EW bug fix

[moustakas]

This PR adds:

• Supporting custom spectrophotometric coadds (both tile-based and healpix-based);
• Fixes #86.
• Updates the atomic data used to set priors on laboratory wavelengths and doublet-ratios, the biggest effect being on the [OII] 7320,30 doublet (which is in fact a doublet or doublets!); specifically, it addresses #84.
• Allows the minimum allowable redshift to be negative, so that low-z galaxies and nebulae in local galaxies like M31 can be fitted.

[coveralls]

Pull Request Test Coverage Report for Build 3300497839

• 17 of 40 (42.5%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 70.255%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/fastspecfit/emlines.py	3	8	37.5%
py/fastspecfit/io.py	7	25	28.0%
<!--	Total:	17	40	42.5%	-->

Totals
Change from base Build 2839287875:	-0.2%
Covered Lines:	2173
Relevant Lines:	3093

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💛 - Coveralls