♻️ Auto-compute certain need fields

[chrisjsewell]

A number of need fields are actually data that is based on other data:

• is_need / is_part are based on whether the item is a NeedItem or NeedPartItem instance
• is_modified is based on if modifications > 0
• id_parent / id_complete are based on the id (and part id if NeedPartItem)
• section_name is the first item in sections
• parent_need is the first item in the parent_needs links
• constraints is the keys of constraints_results
• constraints_passed is derived from constraints_results

These are mainly there for user convenience when writing filters, but are not necessary to specify independently, and could lead to inconcistencies in the data.

Therefore, these fields are now auto-computed when initialiasing/modifying the need, rather than being allowed to be specified / modified independantly.

[codecov]

Codecov Report

❌ Patch coverage is 91.54930% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.79%. Comparing base (4e10030) to head (7dd47fa).
⚠️ Report is 150 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/need_item.py	85.71%	5 Missing ⚠️
sphinx_needs/directives/need.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1496      +/-   ##
==========================================
+ Coverage   86.87%   88.79%   +1.91%     
==========================================
  Files          56       68      +12     
  Lines        6532     8640    +2108     
==========================================
+ Hits         5675     7672    +1997     
- Misses        857      968     +111     

Flag	Coverage Δ
pytests	88.79% <91.54%> (+1.91%)	⬆️

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