2.3.1 release candidate

[moustakas]

This PR includes a variety of fixes and updates and is a 2.3.1 release candidate.

Highlights include:

• Bug fix from 2.3.0 tests when one or more cameras on the first spectrum in a given healpixel is fully masked.
• Restore support for fitting stacked spectra (totally broken in 2.3.0.
• Restore the xtol convergence criterion parameter in scipy.optimize.least_squares from 1e-2 to its default (1e-8) to prevent the optimization from stopping before actual convergence. One example where this matters is sv3-bright-25918-39627758090851408. I also removed the requirement that the final velocity widths and centers cannot change by more than +/-20% during the final round of fitting. I found that objects like sv3-bright-25918-39627758090851408 require the additional flexibility after some of the constraints are relaxed in the final iteration. I will need to do some additional larger-scale tests to ensure this change doesn't introduce other problems, however.
• The _FLUX and _FLUX_IVAR parameters are now derived using the method documented in https://github.com/desihub/fastspecfit/issues/127#issuecomment-1677443903. See the full thread in #127 for additional details and caveats regarding these parameters.
• After investigating some of the objects with broad line-emission documented in #118 and comparing the fitting results with low-mass, narrow-line galaxies with no AGN, I decided to restore the requirement that at least one of the two reddest broad Balmer lines (e.g., Halpha or Hbeta) must be detected with S/N>3. (More documentation and testing to follow.)
• As requested in #112, He II 1640 and Al III 1857 have been added to the default data model. (The 1450, 1700, 3000 A continuum luminosities were added in #145).
• The emission-line parameter file is now an optional input, in order to allow for custom fitting.
• Various documentation and web-app updates.

[coveralls]

Pull Request Test Coverage Report for Build 5859917529

Details

• 167 of 250 (66.8%) changed or added relevant lines in 5 files are covered.
• 407 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 60.0%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/fastspecfit/fastspecfit.py	4	8	50.0%
py/fastspecfit/emlines.py	40	52	76.92%
py/fastspecfit/io.py	20	40	50.0%
py/fastspecfit/continuum.py	101	148	68.24%
<!--	Total:	167	250	66.8%	-->

Files with Coverage Reduction	New Missed Lines	%
py/fastspecfit/emlines.py	1	51.31%
py/fastspecfit/io.py	187	54.72%
py/fastspecfit/continuum.py	219	72.95%
<!--	Total:	407		-->

Totals
Change from base Build 5812793938:	-0.01%
Covered Lines:	2454
Relevant Lines:	4090

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💛 - Coveralls