Fix #8: projection on dictionary fields

This commit allows dictionaries and lists of dictionaries to be projected on a given field.

For instance,

* d = { a: 1, b: 2 }
* a = d.a
* b = d.b

defines a single parameter set where d is the dictionary { a: 1, b: 2 }, and a = 1 and b = 2.

An error is raised if the field is not in the dictionary.

For a list of dictionaries,

* d = [{ a: 1, b: 2 }, { a : 3 }, { b : 4 }]
* a = d.a

a is bound to the list [1, 3] and the dictionary { b : 4 } where a is missing is just ignored without any error.