Writes the Graphviz source out as text. Without it, printing a DOT object
fell to print.default, which showed the whole graph as one escaped
string ([1] "digraph NMF \{\n ...") — unreadable. Shared by every
*.DOT function through the common "nmfkc.DOT" class.
Usage
# S3 method for class 'nmfkc.DOT'
print(x, ...)