This function reads genotype information stored in binary PED (BED) files used in plink.
These files save space and time. The pedigree/phenotype information is stored in a separate file (*.fam)
and the map information is stored in an extededed MAP file (*.bim) that contains information about the allele names,
which would otherwise be lost in the BED file. More details http://pngu.mgh.harvard.edu/~purcell/plink/binary.shtml.
The routine will return a vector of dimension n*p (n=number of individuals, p=number of snps), with the snps(individuals) stacked,
depending whether the BED file is in SNP-major or individual-major mode.
The vector contains integer codes:
ll{
Integer code Genotype
0 00 Homozygote "1"/"1"
1 01 Heterozygote
2 10 Missing genotype
3 11 Homozygote "2"/"2"
}