Add missing docstring for to_image

[gerritholl]

Add missing docstring for the to_image function in the satpy.writers.__init__ module.

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #703 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #703   +/-   ##
=======================================
  Coverage   79.68%   79.68%           
=======================================
  Files         145      145           
  Lines       21191    21191           
=======================================
  Hits        16886    16886           
  Misses       4305     4305

Impacted Files	Coverage Δ
satpy/writers/__init__.py	85.82% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 79.685% when pulling 7a243a6 on gerritholl:add-missing-docsting-to-image into f0d1136 on pytroll:master.

[djhoese]

@mraspaud isn't this method now obsolete since I think the XRImage class itself does all of this? Or does it not squeeze? Is the squeeze needed?

[gerritholl]

I don't know if it's obsolete. I got here by going through #513 where this function is recommended. If it's obsolete then instead of this PR it should be changed in #513 (and maybe a DeprecrationWarning issued here?).

[mraspaud]

Squeeze might be needed to remove some unneeded, but tbh I don't remember

[djhoese]

This is the current stuff that XRImage does to "correct" for various dimension schemes: https://github.com/pytroll/trollimage/blob/master/trollimage/xrimage.py#L189-L220

It doesn't do any squeezing, but I'm wondering if it could be smarter. This may not be the time since this could get complicated (if you have a 5 dimensional DataArray should you be able to make an image out of that?).

[djhoese]

Regardless of whether or not this function is needed, the docstring is useful so I'll merge it. We can figure out how images are created at another time.