View implementation

[davidbrochart]

In #28 we implemented views on zarray with the following limitation: if the zarray holds an expression which is already a view, then this expression is evaluated and stored in an array before getting the new view (if the zarray holds an expression which is not a view, then this expression is not evaluated before getting the new view). We might want to improve this behavior in the future, and "merge" consecutive views into a single one.