Add new Atom type that represents datasinks

[fmagin]

Now that FunctionCall atoms don't exist anymore, it got impossible to model questions like whether a certain argument has a dataflow path to a certain function.

As an alternative way to model this, I propose a new kind of Atom that represents the general idea of a sink.

It might also be interesting to have an Atom for datasources, e.g. if a function reads from a file from the filesystem, a FunctionHandler could add this kind of Atom as a dependency. This requires more code changes though, because LiveDefinitions.get_definitions doesn't support this yet.

Overall this is still a Draft PR and a suggestion, I am currently still experimenting with the code built on top of this idea