Data transformer: Blacklist nested data structures

[KristianLyng]

A common case we need to address is that a number of senders can't handle nested data structures. The proper way to handle it today is to use flatten/split transformers to handle it. However, sometimes, items are overlooked and still sent downstream.

I suggest we make a new function to the data/metadata transformers that run the same checkVariable() check found in senders/influx.go:62 (obviously extracted from module). This will allow us to discard data that will fail to transmit down stream anyway, without going through HTTP post and whatnot.