Node/Edge Attribute read & write inconsistencies

[bernlu]

Writing a string attribute and reading it back does not always result in the same attribute data.

Example:

NetworKit::Graph G(1);
auto stringAttr = G.nodeAttributes().attach<std::string>("stringAttr");
stringAttr[0] = "test with spaces";
stringAttr.write("stringAttr.txt");
stringAttr.read("stringAttr.txt");
std::cout << stringAttr.get(0);
// prints: test

(the specific problem here is that the read function currently stops reading a line after the first whitespace)

I have implemented a quick fix for this for my specific use case (link), but I think we should add some more general tests and make sure it works for most data types that usually occur (at least int/double/string since those are available via python bindings as well, and maybe vectors of those types).