move dev and sandbox to the template repo

[Remi-Gau]

• also update readme to recommend using the PTB experiment template repo to create new experiment

[codecov]

Codecov Report

Merging #132 into dev will increase coverage by 0.22%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #132      +/-   ##
==========================================
+ Coverage   21.57%   21.79%   +0.22%     
==========================================
  Files          55       55              
  Lines         788      789       +1     
==========================================
+ Hits          170      172       +2     
+ Misses        618      617       -1     

Flag	Coverage Δ
#unittests	21.79% <100.00%> (+0.22%)	⬆️

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Impacted Files	Coverage Δ
src/utils/setDefaultsPTB.m	85.41% <100.00%> (+2.43%)	⬆️

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