skip population creation if exists

[egillax]

Closes #425

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (988221c) 89.34% compared to head (8fbee84) 89.35%.

Files	Patch %	Lines
R/RunPlp.R	88.88%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #426      +/-   ##
===========================================
+ Coverage    89.34%   89.35%   +0.01%     
===========================================
  Files           49       49              
  Lines         9829     9830       +1     
===========================================
+ Hits          8782     8784       +2     
+ Misses        1047     1046       -1     

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