Double trapezoidal integral

Double trapezoidal integral over a uniformly spaced interval.
Inputs:
- f : handle to the (vectorial) function to integrate
- x_min : lower bound of the first variable
- x_max : upper bound of the first variable
- y_min : lower bound of the second variable
- y_max : upper bound of the second variable
- n_points : number of points to sample per dimension

Outputs
- out : (vector column) result