create extent attr for GridSpaces

[Tortar]

this spares some indirection

[codecov-commenter]

Codecov Report

Merging #880 (8a16078) into main (e48591d) will increase coverage by 0.82%.
Report is 5 commits behind head on main.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main     #880      +/-   ##
==========================================
+ Coverage   71.88%   72.71%   +0.82%     
==========================================
  Files          42       42              
  Lines        2746     2873     +127     
==========================================
+ Hits         1974     2089     +115     
- Misses        772      784      +12     

Files Changed	Coverage Δ
src/submodules/pathfinding/astar.jl	90.12% <ø> (-3.12%)	⬇️
src/spaces/grid_general.jl	98.33% <66.66%> (-0.51%)	⬇️
src/spaces/grid_multi.jl	83.33% <100.00%> (+1.72%)	⬆️
src/spaces/grid_single.jl	95.74% <100.00%> (+1.15%)	⬆️
src/submodules/io/jld2_integration.jl	100.00% <100.00%> (ø)

... and 10 files with indirect coverage changes

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