$address: address of the external pointer containing the underlying
C++ object, to be used internally as a XPtr<bed> in C++ code
$extptr: use $address instead
$bedfile: path to the bed file
$bimfile: path to the corresponding bim file
$famfile: path to the corresponding fam file
$prefix: path without extension
$nrow: number of samples in the bed file
$ncol: number of variants in the bed file
$map: data frame read from $bimfile
$fam: data frame read from $famfile
$.map: use $map instead
$.fam: use $fam instead
$light: get a lighter version of this object for parallel algorithms
to not have to transfer e.g. $.map.