bug fixed: removing invocation nodes caused npe
adding dataflow connections between methods works!
- TODO: add support for removing connections, distinction between argument and variable (how to support null etc.)
argument api seems to work (passes lang tests)
dataflow ui seems to work for invocations
- (including null args and nested method calls)
- nested invocation does not need control flow!

TODO:

visualize variable dataflow (probably needs rework in dataflow generation, does data relation have to support variables and not only invocations?)
fixed NULL arg error in ast transform (null was treated as regular const arg instead of null arg)
fixed variable declaration in for loops:
- was only done of "dummyVar" in groovy ast transform
- variables are now only referenced in convertArguments() method