Jkmarx/move node related to data set manager

[jkmarx]

Resolves #2837

• Node related code was moved from the core module to the data_set_manager. (No new code was introduced.)
• API V1, API V2, Serializer, API V1 Unit tests

• API V2 Unit test will be written in a separate pull request.

[codecov]

Codecov Report

Merging #2911 into develop will decrease coverage by 0.14%.
The diff coverage is 60.67%.

@@             Coverage Diff             @@
##           develop    #2911      +/-   ##
===========================================
- Coverage    59.33%   59.19%   -0.15%     
===========================================
  Files          435      435              
  Lines        28180    27227     -953     
  Branches      1273     1273              
===========================================
- Hits         16721    16116     -605     
+ Misses       11459    11111     -348

Impacted Files	Coverage Δ
refinery/core/urls.py	100% <ø> (ø)	⬆️
refinery/data_set_manager/urls.py	100% <ø> (ø)	⬆️
refinery/core/serializers.py	84.93% <ø> (+1.89%)	⬆️
refinery/core/api.py	39.24% <100%> (-1.16%)	⬇️
refinery/core/test_api.py	99.16% <100%> (-0.55%)	⬇️
refinery/core/views.py	41.94% <100%> (-2.49%)	⬇️
refinery/data_set_manager/serializers.py	100% <100%> (ø)	⬆️
refinery/data_set_manager/views.py	56.47% <23.07%> (-5.15%)	⬇️
refinery/data_set_manager/api.py	95.37% <87.5%> (-2.58%)	⬇️
... and 8 more

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