✨ Add get_needs_view function to public API

[chrisjsewell]

This allows users to access to the (immutable) resolved needs during the write/analysis phase (for example https://github.com/useblocks/sphinx-modeling, cc @ubmarco, also see #1264 for more info)

To ensure it is not misused, checks are now put in-place to disallow modification of needs after the needs have been resolved.
This actually surfaced a few issues where the resolved needs were mistakenly accessed, instead of the unresolved (mutable) ones.

The other reason for doing this, is that if we "know" we are only accessing immutable needs, then we will be able to improve filtering performance, by performing lazy indexing etc (see #1281)

[codecov]

Codecov Report

Attention: Patch coverage is 75.86207% with 7 lines in your changes missing coverage. Please review.

Project coverage is 86.95%. Comparing base (4e10030) to head (0975799).
Report is 54 commits behind head on master.

Files with missing lines	Patch %	Lines
sphinx_needs/data.py	63.63%	4 Missing ⚠️
sphinx_needs/functions/common.py	33.33%	2 Missing ⚠️
sphinx_needs/api/need.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1277      +/-   ##
==========================================
+ Coverage   86.87%   86.95%   +0.07%     
==========================================
  Files          56       60       +4     
  Lines        6532     6978     +446     
==========================================
+ Hits         5675     6068     +393     
- Misses        857      910      +53     

Flag	Coverage Δ
pytests	86.95% <75.86%> (+0.07%)	⬆️

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[ubmarco]

Can you raise an exception if not in the right phase?

[chrisjsewell]

This has been subsumed by #1281