🔧 Centralise need parts creation and strongly type needs

[chrisjsewell]

Need "parts" (a.k.a. sub-needs) are expanded to full needs by copying the parent node then overding certain fields like the ID and content.
This PR replaces the duplicated logic by a single iter_need_parts function.

It removes the is_need, is_part keys from the parts dict, since this is handled in iter_need_parts,
plus it removes the document key, since this appears to be an erroneous duplication of the needs docname field (the part is always in the same document as the parent need)

Since the NeedsPartsInfoType is now identical to NeedsInfoType we remove it (these have to be identical, to allow filters to work for both)

Lastly the https://docs.python.org/3/library/typing.html#typing.Required is added to core fields in NeedsInfoType,
to allow for add_need to be properly typed checked, and so should ensure future code changes do not break the need schema.

[codecov]

Codecov Report

Attention: Patch coverage is 97.46835% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.92%. Comparing base (166a0e3) to head (a463e51).

Files	Patch %	Lines
sphinx_needs/api/need.py	66.66%	1 Missing ⚠️
sphinx_needs/data.py	98.24%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1129      +/-   ##
==========================================
- Coverage   85.94%   85.92%   -0.02%     
==========================================
  Files          56       56              
  Lines        6537     6531       -6     
==========================================
- Hits         5618     5612       -6     
  Misses        919      919              

Flag	Coverage Δ
pytests	85.92% <97.46%> (-0.02%)	⬇️

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[danwos]

The for the cleanup.
Looks all good :) 👍