Refactor grid graph

[GiggleLiu]

This is for supporting UnitDiskMapping.
In this PR, we refined grid graph as a special case of UnitDiskGraph.

const GridGraph{D} = UnitDiskGraph{D, Int}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.95%. Comparing base (c29dfc1) to head (440f569).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
+ Coverage   94.89%   94.95%   +0.06%     
==========================================
  Files          34       34              
  Lines        1449     1448       -1     
==========================================
  Hits         1375     1375              
+ Misses         74       73       -1     

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[GiggleLiu]

Since all tests pass and I merge this PR directly.