More pythonic messages

[roxanebellot]

In order to make dev of python apps using netorcai easier, here is a few changes, mainly about understandability and readability

Changes

• Pep8 compliance
• all message now have a __repr__ so are human-readable when printed
• list parsage in list comprehension instead of function -> more readable + more efficient

[codecov-io]

Codecov Report

Merging #2 into master will decrease coverage by 5.03%.
The diff coverage is 57.89%.

@@            Coverage Diff            @@
##           master      #2      +/-   ##
=========================================
- Coverage   99.34%   94.3%   -5.04%     
=========================================
  Files           3       3              
  Lines         152     158       +6     
=========================================
- Hits          151     149       -2     
- Misses          1       9       +8

Impacted Files	Coverage Δ
netorcai/message.py	86.88% <57.89%> (-13.12%)	⬇️

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