read.population string concatenations

[mark-de-haan]

At the moment you supply offspring phenotypes by: offspring = "phenotypes.txt"

p2g then adds "_phenotypes.txt" to it, resulting in phenotypes.txt_phenotypes.txt.

p2g then checks if this file exists. Of course this file does not exist because you already supplied the correct file name when calling the function.

This is also the case with founder data, genotype data and map data. If a user inputted option already contains a file extension, you do not need to add another one.

[KonradZych]

parameter offspring specifies a "core" of all the filenames containing offspring data:

offspring + "_phenotypes.txt" -> phenotypic offspring + "_genotypes.txt" -> genotypic

probably we can even think about skipping it, but for now it is correct in my opinion