Allow pack() functions to accept strings as input for as_sources and as_targets args

[YanisLalou]

Currently, every DAdataset packing function is only compatible with List[str] for the as_sources and as_targets arguments. However, in cases where we have only one target and one source, it would be simpler to use a single string instead of a list of strings.
OR, the use of strings altogether could be prohibited.
Right now, when we provide a string instead of a list, the packing functions iterate over the string, letter by letter, eventually raising a KeyError exception."