Add pre save, checking geom has not be changed

[LePetitTim]

No description provided.

[codecov]

Codecov Report

Merging #89 (7b5b271) into master (67baded) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
+ Coverage   95.96%   95.99%   +0.03%     
==========================================
  Files          37       37              
  Lines        1809     1823      +14     
==========================================
+ Hits         1736     1750      +14     
  Misses         73       73              

Impacted Files	Coverage Δ
geostore/models.py	97.97% <100.00%> (+0.09%)	⬆️

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