demonstrator for source/record refactoring

[mki-c2c]

an example how the data sources can be refactored
see #1496

[codecov]

Codecov Report

Attention: Patch coverage is 91.48936% with 4 lines in your changes missing coverage. Please review.

Project coverage is 80.57%. Comparing base (14f2071) to head (a4e324a).
Report is 105 commits behind head on master.

Files	Patch %	Lines
pyramid_oereb/core/sources/__init__.py	86.95%	3 Missing ⚠️
...id_oereb/contrib/data_sources/swisstopo/address.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1928      +/-   ##
==========================================
- Coverage   80.60%   80.57%   -0.04%     
==========================================
  Files         125      124       -1     
  Lines        5265     5260       -5     
==========================================
- Hits         4244     4238       -6     
- Misses       1021     1022       +1     

Flag	Coverage Δ
unittests	80.57% <91.48%> (-0.04%)	⬇️

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