Provide method in ATFCONLConvertor to convert streams/strings

[larsgw]

Currently ATFCONLConvertor only works directly on files, which makes it hard to run the script through standard in. If the class instead had a method for taking input from a stream (and a file name), and a method for writing to a string instead of a file, it would be easier. convert() and write2file() could use those methods internally even.