Creates Group model for post

[NinadBhat]

Fixes #26

[codecov-commenter]

Codecov Report

Merging #29 (836b8cc) into master (ac1ccf3) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #29      +/-   ##
==========================================
+ Coverage   92.09%   92.24%   +0.15%     
==========================================
  Files          25       25              
  Lines         746      761      +15     
==========================================
+ Hits          687      702      +15     
  Misses         59       59              

Flag	Coverage Δ
pytests	92.24% <100.00%> (+0.15%)	⬆️

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Impacted Files	Coverage Δ
aiida_restapi/models.py	95.89% <100.00%> (+1.06%)	⬆️
aiida_restapi/routers/groups.py	100.00% <100.00%> (ø)

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[ltalirz]

@chrisjsewell Just a reminder to please add your review of this PR