create and update rtree spatial index only when needed

[wang-boyu]

Implements #104

Check performance of the gis example models. Execution time is the average of 10 runs, where each run contains model initialization followed by 10 steps.

model	time in seconds	time in seconds (this PR)
agents_networks (num_agents=100)	3.18	0.44
agents_networks (num_agents=500)	37.28	1.16
geo_schelling (default parameters)	0.79	0.62
geo_schelling (points & polygons) (default parameters)	1.49	1.19
geo_sir (default parameters)	0.13	0.13
population (default parameters)	12.88	10.31

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (9dfbe7c) 75.90% compared to head (205f850) 78.14%.

Files	Patch %	Lines
mesa_geo/geospace.py	84.61%	3 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   75.90%   78.14%   +2.23%     
==========================================
  Files          10       10              
  Lines         660      691      +31     
  Branches      138      151      +13     
==========================================
+ Hits          501      540      +39     
+ Misses        139      127      -12     
- Partials       20       24       +4     

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[wang-boyu]

@Corvince @rht would you like to help review this PR? Thanks : )

[Corvince]

Hi, yes, I can do that hopefully tonight or tomorrow!

[Corvince]

Sorry I got some visitors, so it will take me some more days until I have a free evening

[wang-boyu]

No worries! Enjoy your holidays.

[Corvince]

Finally reviewed and merged :)

Good job

[wang-boyu]

Thanks for reviewing! Appreciate it.