String of allowed amino acids
allowed_amino_acids.RdReturns a data.frame of allowed amino acid single-letter codes. These come
from IUPAC (International Union of Pure and Applied Chemistry),
see here for details.
Variants defined in the variant_string column of the counts data.frame
must be one of these single-letter codes, or otherwise the underscore (_),
pipe (|) or forward slash (/) symbols. Underscores will be removed but are
allowed for improved readability. The pipe and forward slash symbols
represent phased or unphased mixed calls, respectively.