fix python3 issue with bytes vs. utf-8

[CellularPhysicalChemistryGroup]

• fixes issues where the amino acid sequence ("seq") is loaded in an ndarray of byte strings. This is denoted as b'AA' where AA would be the three-letter amino acid code. Since the whole file uses utf-8 encoding, I adjusted the code so that the sequence ndarray is converted to utf-8 for each amino acid.