Implement maximum and minimum aggregation function

[denismerigoux]

Catala supports maximum and minimum aggregation functions for collections. Right now, these are only present in the surface syntax.

One has to fill the holes like this one :

catala/src/catala/catala_surface/desugaring.ml

Lines 362 to 364 in 933fd02

	| Ast.Aggregate (Ast.AggregateExtremum _) ->
	Errors.raise_spanned_error "Unsupported feature: minimum and maximum"
	(Pos.get_position op')

To do that, the expression maximum money for salary in salaries of (salary - $200) should desugar to

fold (fun acc salary -> let tmp = (salary - 200) in if acc < tmp then tmp else acc) <INIT> salaries

The problem lies within the <INIT> expression. What to put here? I guess this should be provided by the user, so we have to update the syntax to something like maximum money context $0 for salary in salaries of (salary - $200)

[nchataing]

What should the extremum of an empty collection returns ? If it raises an error, then we could take any element of the collection for the <INIT> expressions.

[denismerigoux]

In case of an empty collection, it should return <INIT> precisely, that's why we need an <INIT>

[denismerigoux]

I'm not confortable with forcing collections to be non-empty. Then it would force us to wrap collections in option types, which is cumbersome.