Fix 'seviri_0deg' and 'seviri_iodc' builtin areas (areas.yaml) not matching reader areas

[sfinkens]

Make sure the extents of the seviri* area defs in areas.yaml are consistent with the extent of the area def returned by the seviri_l1b_hrit reader.

• Tests passed

[coveralls]

Coverage increased (+0.005%) to 79.777% when pulling 44316c6 on sfinkens:align-seviri-areas-yaml into f422003 on pytroll:master.

[codecov]

Codecov Report

Merging #708 into master will increase coverage by 0.26%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #708      +/-   ##
==========================================
+ Coverage   79.77%   80.04%   +0.26%     
==========================================
  Files         147      148       +1     
  Lines       21347    21923     +576     
==========================================
+ Hits        17030    17548     +518     
- Misses       4317     4375      +58

Impacted Files	Coverage Δ
satpy/demo/__init__.py	94.54% <0%> (-0.91%)	⬇️
satpy/readers/viirs_edr_active_fires.py	89.55% <0%> (-0.45%)	⬇️
satpy/demo/_google_cloud_platform.py	95.45% <0%> (ø)
satpy/tests/compositor_tests/__init__.py	99.46% <0%> (+0.12%)	⬆️
satpy/tests/test_demo.py	96% <0%> (+2.15%)	⬆️
satpy/composites/__init__.py	70.92% <0%> (+5.37%)	⬆️

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