Returns a data frame which contains the following columns:
speciesThe species name as given by the corresponding folder
name in the repository (e.g. Yersinia\_pestis\_KIM).
accessionThe accession number as given by the common prefix
of file names in the repository (e.g. NC\_004088).
size.bpThe size of the sequence in bp (e.g. 4600755).
typeA factor with two levels (plasmid or chromosome) temptatively
deduced from the description of the sequence.