Changed GraphGroup constructor from Internal to Public

[danielpastoor]

Error when trying to get group data from a team site:
No parameterless constructor defined for type 'PnP.Core.Model.Security.GraphGroup'

Fix:
Change the constructor of GraphGroup from Internal to Public.

And added the documentation for the fixed part.

[jansenbe]

Nice catch on the constructor @danielpastoor , it was accidently left internal.

[codecov-commenter]

Codecov Report

Merging #845 (a836e7c) into dev (63545f3) will increase coverage by 0.86%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##              dev     #845      +/-   ##
==========================================
+ Coverage   82.42%   83.29%   +0.86%     
==========================================
  Files         416      522     +106     
  Lines       28590    35034    +6444     
==========================================
+ Hits        23565    29180    +5615     
- Misses       5025     5854     +829     

Impacted Files	Coverage Δ
...re/Model/SharePoint/Changes/Internal/ChangeUser.cs	0.00% <0.00%> (-100.00%)	⬇️
.../PnP.Core/Model/SharePoint/Core/Internal/Folder.cs	73.28% <0.00%> (-23.89%)	⬇️
...odel/Security/Internal/SharePointUserCollection.cs	78.78% <0.00%> (-21.22%)	⬇️
....Core/Exceptions/MicrosoftGraphServiceException.cs	80.00% <0.00%> (-20.00%)	⬇️
...PnP.Core.Admin/Model/Teams/Internal/TeamCreator.cs	76.19% <0.00%> (-14.29%)	⬇️
src/sdk/PnP.Core/Services/Core/TestManager.cs	54.94% <0.00%> (-12.96%)	⬇️
.../Model/SharePoint/Core/Internal/SharePointAdmin.cs	81.06% <0.00%> (-12.80%)	⬇️
.../SharePoint/Core/Internal/ContentTypeCollection.cs	87.67% <0.00%> (-12.33%)	⬇️
...e/Model/Teams/Internal/TeamChannelTabCollection.cs	87.92% <0.00%> (-12.08%)	⬇️
.../Model/SharePoint/Core/Internal/ListItemVersion.cs	66.66% <0.00%> (-11.12%)	⬇️
... and 193 more

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