Fix wrong number of scanlines in eps reader

[mraspaud]

Sometimes the number of declared scanlines in the eps l1b files is different from the number of MDR records. This PR ensures we use the number of records as truth.

• Closes AVHRR lat/lon grids incorrect size #1367
• Tests added
• Passes flake8 satpy

@TalfanBarnie Can you test if this works for you?

[coveralls]

Coverage increased (+0.05%) to 90.517% when pulling 1571c9d on mraspaud:fix_eps_number_of_scanlines into d728506 on pytroll:master.

[codecov]

Codecov Report

Merging #1368 into master will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1368      +/-   ##
==========================================
+ Coverage   90.46%   90.52%   +0.05%     
==========================================
  Files         228      228              
  Lines       33217    33334     +117     
==========================================
+ Hits        30050    30174     +124     
+ Misses       3167     3160       -7     

Impacted Files	Coverage Δ
satpy/readers/eps_l1b.py	76.13% <100.00%> (+2.25%)	⬆️
satpy/tests/reader_tests/test_eps_l1b.py	100.00% <100.00%> (ø)
satpy/tests/writer_tests/test_cf.py	99.64% <0.00%> (+0.02%)	⬆️
satpy/scene.py	90.59% <0.00%> (+0.17%)	⬆️
satpy/writers/cf_writer.py	94.09% <0.00%> (+0.20%)	⬆️

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[mraspaud]

Confirmed to work from @TalfanBarnie who reported #1367

[pnuu]

LGTM. Few suggestions and one question inline.