Change VLStringAtom to bytestring
Change VLStringAtom to bytestring

Only allow bytestring in VLStringAtom(). Update docstring.

FrancescAlted/PyTables,avalentino/PyTables,avalentino/PyTables,FrancescAlted/PyTables,PyTables/PyTables,PyTables/PyTables,PyTables/PyTables,avalentino/PyTables,FrancescAlted/PyTables