This is an internal function that returns the locations of genotype dimensions from the genotype object. It is a relic from when DOQTL put loci in the second dimension and alleles in the third, while R/qtl2 put loci in the third dimension and alleles in the second.
get_geno_dim()
Returns a vector of three named elements identifying which dimension the individual (mouse), allele, and loci are in.