Identity function.
If p is the returned Legendre object, then p(x) == x for all values of x.
domain : array_like
The resulting array must be of the form [beg, end], where beg and end are the endpoints of the domain.
window : array_like
The resulting array must be if the form [beg, end], where beg and end are the endpoints of the window.
identity : Legendre instance