Consistent platform id attribute across NAT + HRIT SEVIRI readers

[simonrp84]

Bugfix
Currently, the SEVIRI L1B Native reader will crash, as it is unable to calibrate IR channels:
AttributeError: 'NativeMSGFileHandler' object has no attribute 'platform_id'
This is because the Native reader stores platform_id inside self.mda whereas the calibration routine expects it as self.platform_id. The HRIT reader stores this attribute as self.platform_id.

This PR updates the Native reader to store the attribute as self.platform_id, thus making both SEVIRI readers consistent and preventing the AttributeError from occurring.

[edit] The method described in issue #722 is a better solution to this.

• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[codecov]

Codecov Report

Merging #721 into master will increase coverage by <.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
+ Coverage   80.01%   80.02%   +<.01%     
==========================================
  Files         147      147              
  Lines       21498    21498              
==========================================
+ Hits        17202    17203       +1     
+ Misses       4296     4295       -1

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_native.py	61.33% <66.66%> (ø)	⬆️
satpy/scene.py	89.89% <0%> (+0.18%)	⬆️

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[coveralls]

Coverage increased (+0.005%) to 80.021% when pulling da3f4dd on simonrp84:seviri_native_cal into 88c0f63 on pytroll:master.

[djhoese]

If @sjoro is fine with this then so am I. (sounds like it from what I got from slack)

[sjoro]

looks good to me.