🧪 Added nested needs test with schema

[ubmarco]

Show how to use nested needs and how schema validation can be applied to enforce need nesting.
Also shows that parent_needs is a link field that can be target of network schema validation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.37%. Comparing base (4e10030) to head (640e189).
⚠️ Report is 228 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1622      +/-   ##
==========================================
+ Coverage   86.87%   88.37%   +1.49%     
==========================================
  Files          56       70      +14     
  Lines        6532     9937    +3405     
==========================================
+ Hits         5675     8782    +3107     
- Misses        857     1155     +298     

Flag	Coverage Δ
pytests	88.37% <ø> (+1.49%)	⬆️

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