Remove dependency on nc_nwcsaf_msg

[mraspaud]

This PR fixes reading the NWCSAF RDT product through using the generic nc_nwcsaf reader instead of the legacy nc_nwcsaf_msg. The latter was removed at the same time.

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

[coveralls]

Coverage increased (+0.2%) to 74.445% when pulling 7f99d46 on mraspaud:fix-nwcsaf-rdt into 6fc2669 on pytroll:master.

[coveralls]

Coverage increased (+0.06%) to 74.318% when pulling adf420a on mraspaud:fix-nwcsaf-rdt into 6fc2669 on pytroll:master.

[codecov]

Codecov Report

Merging #506 into master will increase coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #506      +/-   ##
==========================================
+ Coverage   74.25%   74.44%   +0.18%     
==========================================
  Files         137      136       -1     
  Lines       18218    18149      -69     
==========================================
- Hits        13528    13511      -17     
+ Misses       4690     4638      -52

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