unify functions definition with no arguments

[gdementen]

In version 0.10, one can define functions without arguments with or without parentheses:

func:
    - code
# or
func():
    - code

We should deprecate defining a function without the parentheses, but given that all models ever produced will need to be modified, we need to provide an automatic migration tool. I expect to make this a warning in 0.11, and make it an error in 0.13 at the earliest.