Some functions are dynamically typed

[hadrienk]

Some functions are supposed to return different types depending on the types of the parameters.
We deal with this adjusting the expression type in the expression visitor (see + - * / operators) or in the native function visitor (see nvl) but other cases are not yet handled; abs, log, power and trunc.

The fact that we need to make this gymnastic should be enough to refactor the function abstraction as extending the expression interfaces.

[hadrienk]

@eivindgi, @pawbu, @takvamborgen do you want to take a look at this?

[hadrienk]

https://jira.ssb.no/browse/KP-3070

[hadrienk]

Actually, we should match functions with the names and the arguments' types.