hlaAASeqClass: Class of HLA Amino Acid Sequence Type
In HIBAG: HLA Genotype Imputation with Attribute Bagging

The definition of a class for HLA protein amino acid sequences.

There are following components:

`locus`	HLA locus
`pos.start`	the starting position in basepair
`pos.end`	the end position in basepair
`value`	a data frame
`assembly`	the human genome reference, such like "hg19"
`start.position`	the start position
`reference`	reference sequence

The component value includes:

`sample.id`	sample ID
`allele1`	amino acid or nucleotide sequence
`allele2`	amino acid or nucleotide sequence
`P1, ..., Pn`	if applicable, a matrix of posterior probability, row – sample, column – position of amino acid