Introduced EntityCollection class and its aliases Filter and Config

[ygorelik]

Introduced EntityCollection class and its aliases Filter and Config. Added Python test cases to cover introduced code.

• Fixes Path API should support multiple top data nodes in RPC replies #657. Fixes CRUD service should handle multiple objects #121. Fixes NETCONF service should handle multiple objects #123

[codecov]

Codecov Report

Merging #719 into master will increase coverage by 0.13%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##           master    #719      +/-   ##
=========================================
+ Coverage   79.47%   79.6%   +0.13%     
=========================================
  Files         172     173       +1     
  Lines       16139   16395     +256     
  Branches     1433    1471      +38     
=========================================
+ Hits        12826   13051     +225     
- Misses       2969    2987      +18     
- Partials      344     357      +13

Impacted Files	Coverage Δ
sdk/python/core/ydk/types/__init__.py	100% <100%> (ø)	⬆️
sdk/python/core/tests/test_sanity_netconf.py	97.05% <100%> (+0.61%)	⬆️
sdk/python/core/tests/test_sanity_path.py	99.2% <100%> (+0.05%)	⬆️
sdk/python/core/ydk/models/__init__.py	100% <100%> (ø)	⬆️
...hon/core/tests/test_sanity_type_mismatch_errors.py	87.77% <100%> (ø)	⬆️
sdk/python/core/ydk/services/netconf_service.py	72.11% <68.42%> (-1.45%)	⬇️
sdk/python/core/ydk/services/crud_service.py	85% <71.42%> (-15%)	⬇️
sdk/python/core/ydk/types/py_types.py	78.8% <77.5%> (-0.41%)	⬇️
sdk/python/core/tests/test_ydk_types.py	97.8% <97.8%> (ø)
... and 2 more

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[ghost]

Thanks @ygorelik !