Function Interface: Source Target
A simple implementation, that adds the option to write the result of function calls into a new column and adresses #81 (closed).
This works without altering the function interface itself and instead simply copies the field into a new column called target before the function is invoked.
I initially thought, this could be some sort of intermediate solution to get things rolling without actually touching most of the code base, but with the prospect to eventually change all function signatures and remove this again. But the more I think about it, the more I guess this could be an appropriate solution in itself. In the end, it saves as from adding another argument to the function interface and the explicit checking/copying within all registered function.
