DOC: Refer to PyTroll coding guidelines

[gerritholl]

In the documentation Developer Guide, refer to the general PyTroll coding guidelines. This replaces the much less complete paragraph for SatPy coding guidelines.

[djhoese]

I am ok merging this, but only after the linked to guidelines are updated to reflect current practices:

1. 120 max line length (I think we've completely gotten rid of 80 as a limit)
2. The "python3" compatibility could probably just be removed at this point since most/all projects will drop python 2 support by the end of 2019.
3. I think @mraspaud's maintainability guidelines are too strict for the average developer. IMO they end up making the code needlessly complex/long when they're supposed to do the opposite. Perhaps emphasizing limiting the number of the things mentioned (lines of code, parameters, etc) would be less scary? Or maybe start the section out with nicer language. I understand guidelines are guidelines, but still.
4. Mention "flake8" as a tool for checking style in style checking. Could also mention stickler which is used in most repositories PRs.
5. Remove link to releasing instructions and/or mention "RELEASING.md" instructions that should be in most if not all repositories.

[codecov]

Codecov Report

Merging #659 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #659   +/-   ##
=======================================
  Coverage   78.53%   78.53%           
=======================================
  Files         138      138           
  Lines       20073    20073           
=======================================
  Hits        15765    15765           
  Misses       4308     4308

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[coveralls]

Coverage increased (+0.005%) to 78.538% when pulling a5e21ad on gerritholl:dev-guide-reference into 8eea7cf on pytroll:master.