Add remote reading for hrit seviri

[mraspaud]

This PR adds remote reading to hrit seviri.
Also some cleanup to the native tests.

• Closes #xxxx
• Tests added

[codecov]

Codecov Report

Attention: Patch coverage is 92.43697% with 9 lines in your changes missing coverage. Please review.

Project coverage is 96.14%. Comparing base (14c9f38) to head (17cf0ce).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/hrit_base.py	57.14%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3064      +/-   ##
==========================================
+ Coverage   96.11%   96.14%   +0.03%     
==========================================
  Files         383      383              
  Lines       55685    55785     +100     
==========================================
+ Hits        53523    53636     +113     
+ Misses       2162     2149      -13     

Flag	Coverage Δ
behaviourtests	3.88% <0.00%> (-0.01%)	⬇️
unittests	96.24% <92.43%> (+0.03%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 13314550823

Details

• 110 of 119 (92.44%) changed or added relevant lines in 7 files are covered.
• 8 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 96.252%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/readers/hrit_base.py	12	21	57.14%

Files with Coverage Reduction	New Missed Lines	%
satpy/readers/utils.py	1	94.5%
satpy/readers/hrit_base.py	7	86.6%

Totals
Change from base Build 13294715027:	0.03%
Covered Lines:	53883
Relevant Lines:	55981

---

💛 - Coveralls

[sfinkens]

Awesome, thanks a lot! Nice and clean tests 👍 Just two little questions