Proper handeling of `str` as node vs. cell everywhere

[ffl096]

strs are iterable and thus at many places pass as cells (e.g., "test" being the cell ("t", "e", "s", "t")). This is usually not what is desired. A strshould consistently be considered as one node and passing it to cell functions should be an error. If the user wants "test" to connect the nodes of each letter, they explicitly should use a tuple.