This function is a replacement version of subset.cross that should be
used if the cross object contains any or all of the components "co.located",
"seg.distortion" and "missing" created by a
pullCross call. For a given ind, the function calls
subset.cross to ensure that all elements created from calls to
native R/qtl functions are subsetted appropriately. In addition,
the "co.located", "seg.distortion" and "missing"
elements are also subsetted and if components "seg.distortion"
and "missing" exist, statistics in their respective tables are
recalculated.
It provides identical functionality to subset.cross
with the exception that ind can only be a logical or numeric
vector.