Fix Scene modifying user-provided reader_kwargs

[djhoese]

While working with the MultiScene I noticed that only the first Scene creation was working and failing on the second because reader_kwargs passed by me were being modified by the Scene. This is the simplest fix I could think of for fixing this.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2335 (1d8a03a) into main (cfe4fa9) will increase coverage by 0.00%.
The diff coverage is 59.25%.

@@           Coverage Diff           @@
##             main    #2335   +/-   ##
=======================================
  Coverage   94.58%   94.59%           
=======================================
  Files         314      314           
  Lines       47518    47526    +8     
=======================================
+ Hits        44943    44955   +12     
+ Misses       2575     2571    -4     

Flag	Coverage Δ
behaviourtests	4.50% <3.70%> (-0.01%)	⬇️
unittests	95.21% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/utils.py	24.92% <8.33%> (-0.15%)	⬇️
satpy/tests/test_scene.py	99.50% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Coverage increased (+0.009%) to 95.17% when pulling 1d8a03a on djhoese:bugfix-mod-reader-kwargs into cfe4fa9 on pytroll:main.

[pnuu]

Codecov seems to think the fix isn't covered by the modified tests.

[djhoese]

I never liked codecov anyway 😉

If I click on the link I get this:

But then if I expand utils.py it looks like it isn't being covered:

[djhoese]

I don't know exactly what codecov's problem is. Everything is tested now and when you click on the codecov Details it shows that things are covered...I think.

As discussed on slack with @pnuu, this PR now also clarifies/fixes the case where storage_options are defined at the "global" reader_kwargs level and the per-reader level (use global as defaults, overwrite with reader-specific).

[pnuu]

LGTM