Fix eps_l1b reader Delayed usage causing docs failures

[djhoese]

This was discussed on slack and first seen failing in #2699. Readthedocs builds were failing with a new version of dask. Sphinx apidocs were trying to analyze if a method was a staticmethod and failing with:

  File "/home/docs/checkouts/readthedocs.org/user_builds/satpy/conda/2699/lib/python3.10/site-packages/sphinx/ext/autodoc/__init__.py", line 2159, in import_object
    inspect.isstaticmethod(obj, cls=self.parent, name=self.object_name)):
  File "/home/docs/checkouts/readthedocs.org/user_builds/satpy/conda/2699/lib/python3.10/site-packages/sphinx/util/inspect.py", line 214, in isstaticmethod
    if meth:
  File "/home/docs/checkouts/readthedocs.org/user_builds/satpy/conda/2699/lib/python3.10/site-packages/dask/delayed.py", line 645, in __bool__
    raise TypeError("Truth of Delayed objects is not supported")
TypeError: Truth of Delayed objects is not supported

I was able to reproduce it locally with the newest dask and discovered that it was the eps_l1b.py reader. This reader declared a regular class method as a Delayed object. This is generally considered bad practice as it makes the Delayed object non-serializable in distributed environments/workflows. Technically making it a staticmethod should have fixed it but when I tried it locally I couldn't make it work. I instead decided to make it a global function as there was no real reason to tie it to the class. This seems to fix it.

I then checked trollimage which does use delayed staticmethods and it doesn't fail, but that's because the staticmethod isn't made delayed until it is called. For example res = dask.delayed(self._some_static_method)(a, b, c). This type of calling seems fine.

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e638f9d) 95.39% compared to head (9707e70) 95.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2700   +/-   ##
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  Coverage   95.39%   95.39%           
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  Files         371      371           
  Lines       52690    52690           
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  Hits        50263    50263           
  Misses       2427     2427           

Flag	Coverage Δ
behaviourtests	4.16% <0.00%> (ø)
unittests	96.00% <100.00%> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 7250373855

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on bugfix-delayed-epsl1b at 95.955%

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Totals
Change from base Build 7250054591:	96.0%
Covered Lines:	50389
Relevant Lines:	52513

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💛 - Coveralls

[mraspaud]

LGTM, thanks for the fix!