Change niche flavor to cellcharter_simple and default distance = 3

[marcovarrone]

Description

• Change flavor option in calculate_niche to cellcharter_simple` for CellCharter
• Default distance = 3 for cellcharter_simple.

How has this been tested?

pytest runs successfully.
However, I see there are no unit tests for calculate_niche with cellcharter (now cellcharter_simple).

Closes

Closes #977

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.56%. Comparing base (adba136) to head (69d7d1b).
⚠️ Report is 173 commits behind head on main.

Files with missing lines	Patch %	Lines
src/squidpy/gr/_niche.py	14.28%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
- Coverage   67.67%   66.56%   -1.12%     
==========================================
  Files          40       40              
  Lines        5863     6048     +185     
  Branches     1114     1012     -102     
==========================================
+ Hits         3968     4026      +58     
- Misses       1596     1663      +67     
- Partials      299      359      +60     

Files with missing lines	Coverage Δ
src/squidpy/_constants/_constants.py	100.00% <100.00%> (ø)
src/squidpy/gr/_niche.py	34.86% <14.28%> (+20.34%)	⬆️

... and 10 files with indirect coverage changes

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