Add new spatial graph features and tests (closes #901)

[luckyy14]

This PR implements new spatial graph features and associated tests as discussed in issue #901.

[codecov]

Codecov Report

❌ Patch coverage is 87.09677% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.26%. Comparing base (538427b) to head (d685f3c).

Files with missing lines	Patch %	Lines
src/squidpy/gr/_spagft.py	83.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1091      +/-   ##
==========================================
+ Coverage   66.17%   66.26%   +0.09%     
==========================================
  Files          44       45       +1     
  Lines        7160     7191      +31     
  Branches     1218     1223       +5     
==========================================
+ Hits         4738     4765      +27     
- Misses       1944     1946       +2     
- Partials      478      480       +2     

Files with missing lines	Coverage Δ
src/squidpy/_constants/_constants.py	100.00% <100.00%> (ø)
src/squidpy/gr/_ppatterns.py	78.48% <100.00%> (+0.55%)	⬆️
src/squidpy/gr/_spagft.py	83.33% <83.33%> (ø)

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