update ifu docs to describe about coord_system and pixel scale

[mperrin]

Improve docs to address some of the questions from that help desk ticket.

[obi-wan76]

Looks good!

[mperrin]

Huh. A few of the tests for this docs PR fail, in a way that’s completely unrelated to docs, which is the only thing that changed in this PR. Must be some kind of upstream change or dependency issue?? FYI @obi-wan76 @BradleySappington
Maybe one of you could investigate

[BradleySappington]

@mperrin, looks like photutils updated to version 2.0.2, last successful run it was 1.13.0

[BradleySappington]

@mperrin - I pushed updates to develop that I believe should fix this issue. If you would please merge develop into your update_ifu_docs repo, this pr should fall out and pass all tests. I also raised an issue for the cause of this failure that will need to be addressed separately.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.15%. Comparing base (490d6bf) to head (2dc5d45).
Report is 19 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #923   +/-   ##
========================================
  Coverage    62.15%   62.15%           
========================================
  Files           15       15           
  Lines         7016     7016           
========================================
  Hits          4361     4361           
  Misses        2655     2655           

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