Adjusted feature calculation methods to updated sequtil modules
Added dipeptide feature calculation function
Added dipeptide protein feature category
Bugfixes to get dipeptide composition to work
Added feature categories ss sa codon comp
Bugfix in getting feature category id name dict
Bugfix: check if splitted param str is list with empty sequence
Use amino acid letters of aa composition names
Change getter names sa and ss sequence
Adjusted protein getter names sa ss sequence
Starting to add quasi-sequence-order descriptors