Add exponential kernel to Graph

[knaaptime]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.4%. Comparing base (27776c3) to head (d52d684).
Report is 39 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #761   +/-   ##
=====================================
  Coverage   85.4%   85.4%           
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  Files        150     150           
  Lines      15975   15989   +14     
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+ Hits       13641   13655   +14     
  Misses      2334    2334           

Files	Coverage Δ
libpysal/graph/_kernel.py	82.4% <100.0%> (+0.4%)	⬆️
libpysal/graph/tests/test_kernel.py	99.1% <100.0%> (+<0.1%)	⬆️

... and 2 files with indirect coverage changes

[knaaptime]

should these all probably have formulas here?

[martinfleis]

Ideally

[ljwolf]

Looks great to me!

[knaaptime]

no way to bound it at the bandwidth if you want though 🤷‍♂️. would require another argument

[martinfleis]

Thinking about this more, I am actually not sure if I like passing alpha as bandwidth. It is inconsistent with the rest of the kernels then. Also, distance band has both bandwidth and alpha arguments (though not used together), so we shall aim for some degree of consistency between these.