fix AttributeError in GeoSpace.agents_at()

[wang-boyu]

Fix #161.

[codecov]

Codecov Report

Patch coverage: 87.50% and project coverage change: +2.84% 🎉

Comparison is base (66831b6) 73.05% compared to head (a111fbb) 75.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   73.05%   75.90%   +2.84%     
==========================================
  Files          10       10              
  Lines         657      660       +3     
  Branches      136      138       +2     
==========================================
+ Hits          480      501      +21     
+ Misses        156      139      -17     
+ Partials       21       20       -1     

Files Changed	Coverage Δ
mesa_geo/geospace.py	77.22% <87.50%> (+10.55%)	⬆️

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