Fix pixel region index retrieval

[duytnguyendtn]

When a u.pix defined region attempted to extract_region on a Spectrum1D defined with a spectral axis defined with u.pix, the region index bound detection algorithm failed to resolve the correct indices, instead returning the actual value of the pixel quantity of the spectral region. This resulted in an empty list being returned due to the slicing indices being incorrect.

Quick test snippet:

import astropy.units as u
flux_unit = u.dimensionless_unscaled
spec_unit = u.pix

import numpy as np
from specutils import Spectrum1D
flux = np.random.randn(200)*flux_unit
wavelength = np.arange(5100, 5300)*spec_unit
spec1d = Spectrum1D(spectral_axis=wavelength, flux=flux)

from specutils import SpectralRegion
from specutils.manipulation import extract_region
region = SpectralRegion.from_center(center=5200.5*spec_unit, width=100*spec_unit)
extract_region(spec1d, region)

should return:

Spectrum1D (length=101)
flux:             [ -0.14648, ..., 0.0035672 ],  mean=0.036819
spectral axis:    [ 5150.0 pix, ..., 5250.0 pix ],  mean=5200.0 pix

whereas on main:

Spectrum1D (length=0)
flux:             [ ],  mean= n/a
spectral axis:    [ ],  mean= n/a

tox test failures appear to be unrelated, possibly related to the same issues we've seen in spacetelescope/jdaviz#1901. Related tests are all testing locally

[pllim]

As discussed, this could use more tests to cover edge cases.

What if region is completely out of bounds? Only 1 pix overlap? Within bounds but somehow falls between the gaps of sampled points?

[pllim]

If you rebase after #1002 is merged, the CI should be green again unless this PR broke something.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.98%. Comparing base (c9633e6) to head (9aa0dca).
Report is 151 commits behind head on main.

Files with missing lines	Patch %	Lines
specutils/manipulation/extract_spectral_region.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1001      +/-   ##
==========================================
+ Coverage   69.93%   69.98%   +0.04%     
==========================================
  Files          64       64              
  Lines        4321     4331      +10     
==========================================
+ Hits         3022     3031       +9     
- Misses       1299     1300       +1     

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[rosteen]

Mostly looks good, left a couple comments. Mainly I think the check on ascending pixel axis should go in SpectralAxis.__new__ rather than appearing twice here.

[rosteen]

LGTM now, thanks!

[duytnguyendtn]

Thanks Ricky and Pey-Lian!