Implement operator `()` with slices

[schmouk]

This operator will be equivalent to Python operator [ ] with slices, i.e. with signatures :

operator() (const size_t end)
operator() (const size_t start, const size_t end)
operator() (const size_t start, const size_t end, const size_t step)

[schmouk]

Completed. Still to be tested.