Unify Dataset attribute naming in SEVIRI L2 BUFR-reader

[sjoro]

Unify Dataset attribute naming in SEVIRI L2 BUFR-reader to match other Satpy readers. Change satellite_name to platform_name.

• Tests passed
• Passes flake8 satpy

[mraspaud]

LGTM

[coveralls]

Coverage remained the same at 87.338% when pulling 8c356b5 on sjoro:unify_attrs into 84c74be on pytroll:master.

[codecov]

Codecov Report

Merging #1041 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #1041   +/-   ##
=======================================
  Coverage   87.34%   87.34%           
=======================================
  Files         188      188           
  Lines       28770    28770           
=======================================
  Hits        25128    25128           
  Misses       3642     3642

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_seviri_l2_bufr.py	93.65% <100%> (ø)	⬆️
satpy/readers/seviri_l2_bufr.py	92.18% <100%> (ø)	⬆️

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