Enhance writer.xml to round-trip DataMessages

[khaeru]

Will close #7.

• Complete new compare() methods.
• Document new compare() methods.
• Test >1 other DataMessage; ensure writer.xml gives intelligible/NotImplemented errors for unsupported messages.
• Update What's new.

[codecov]

Codecov Report

Merging #13 into master will increase coverage by 0.06%.
The diff coverage is 97.72%.

@@            Coverage Diff             @@
##           master      #13      +/-   ##
==========================================
+ Coverage   94.06%   94.12%   +0.06%     
==========================================
  Files          51       51              
  Lines        4063     4139      +76     
==========================================
+ Hits         3822     3896      +74     
- Misses        241      243       +2     

Impacted Files	Coverage Δ
sdmx/tests/test_model.py	100.00% <ø> (ø)
sdmx/message.py	97.53% <90.00%> (-1.10%)	⬇️
sdmx/writer/xml.py	98.21% <98.30%> (+0.01%)	⬆️
sdmx/model.py	96.66% <100.00%> (+0.02%)	⬆️
sdmx/reader/sdmxml.py	97.01% <100.00%> (ø)
sdmx/tests/data/__init__.py	97.82% <100.00%> (ø)
sdmx/tests/writer/test_writer_xml.py	100.00% <100.00%> (ø)
sdmx/writer/base.py	95.23% <0.00%> (ø)

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