Returns the subset of the ChipInfo object for which SNPs are on the chromosome specified, by either number or character.
# S4 method for ChipInfo,ANY,ANY
[[(x, i, j, ...)
a ChipInfo object
a chromosome number or letter, i.e, one of seqlevels(x)
always leave missing, not applicable for this method.
further arguments - again there should not be any
ChipInfo object for the subset of SNPs on chromosome i