Improve `load` behavior for protein data
load assumes that when a protein file is provided, the headers in the protein and nucleotide file are the same. The handling of protein data with
--generic is not explicitly described.
It should be possible to load protein data with generic headers, with or without simultaneously loading nucleotide data.
--generic is specified and both nucleotide and protein data are being loaded, it should be assumed that the sequences are in the same order in the two files. This will require using ordered dictionaries to store protein and nucleotide data.