Ensure reproject_and_coadd handles bg-matching with one input image

[svank]

I'm using reproject_and_coadd in a loop---in some iterations, there's only one input image (while in others, there are multiple). When I set match_background=True, I get some warnings and the whole output image is nan. Maybe this indicates a deeper problem in the background-matching code, but as a workaround, we can just not do the bg-matching if there's only a single input image, to work around this bug and avoid some extra work.

import numpy as np
import reproject
import reproject.mosaicking
import matplotlib.pyplot as plt
from astropy.wcs import WCS

data = np.ones((100, 100))
data[::5] = 2
wcs_in = WCS(naxis=2)
wcs_in.wcs.ctype = 'RA---TAN', 'DEC--TAN'
wcs_in.wcs.crpix = 50, 50
wcs_in.wcs.crval = 0, 0
wcs_in.wcs.cdelt = .1, .1

wcs_out = wcs_in.deepcopy()

reprojected, footprint = reproject.mosaicking.reproject_and_coadd(
    [(data, wcs_in)], wcs_out, data.shape,
    reproject_function=reproject.reproject_adaptive, match_background=True)

/home/svankooten/.anaconda/envs/sunpy-dev/lib/python3.11/site-packages/numpy/core/fromnumeric.py:3464: RuntimeWarning: Mean of empty slice.
  return _methods._mean(a, axis=axis, dtype=dtype,
/home/svankooten/.anaconda/envs/sunpy-dev/lib/python3.11/site-packages/numpy/core/_methods.py:192: RuntimeWarning: invalid value encountered in scalar divide
  ret = ret.dtype.type(ret / rcount)
/home/svankooten/Documents/Research/reproject_dev/reproject/reproject/mosaicking/background.py:132: RuntimeWarning: Mean of empty slice
  corrections -= np.nanmean(corrections)

np.all(np.isnan(reprojected)) # Output: True

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Coverage   93.62%   93.62%           
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  Files          25       25           
  Lines         894      894           
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  Hits          837      837           
  Misses         57       57           

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[astrofrog]

Looks good to me! Background matching should be a noop with one image so this is the right fix.

[astrofrog]

Failures are unrelated