add documentation for reading lower-dimensional objects

[keflavich]

Solves #833

I smuggled in a configuration item - ext=0 is now a keyword arg instead of hard-coding it.

[codecov-commenter]

Codecov Report

Merging #834 (ede3260) into master (bebd96a) will increase coverage by 0.30%. The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #834      +/-   ##
==========================================
+ Coverage   77.87%   78.17%   +0.30%     
==========================================
  Files          24       24              
  Lines        5863     5921      +58     
==========================================
+ Hits         4566     4629      +63     
+ Misses       1297     1292       -5     

Impacted Files	Coverage Δ
spectral_cube/lower_dimensional_structures.py	80.62% <75.00%> (ø)
spectral_cube/io/casa_image.py	66.05% <0.00%> (ø)
spectral_cube/spectral_cube.py	76.53% <0.00%> (+0.04%)	:arrow_up:
spectral_cube/io/fits.py	86.62% <0.00%> (+0.17%)	:arrow_up:
spectral_cube/conftest.py	93.49% <0.00%> (+0.72%)	:arrow_up:
spectral_cube/io/core.py	90.90% <0.00%> (+1.51%)	:arrow_up:
spectral_cube/cube_utils.py	83.07% <0.00%> (+1.63%)	:arrow_up:

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[keflavich]

This probably ought to be squashed on merge because of all the gh-suggest commits

[keflavich]

afaict, the error is unrelated. I'm gonna merge and hope the windows-based, dask-test error just goes away...