2D trapezoidal rule

Calculates a double integral using trapezoidal rule.

This is a very simple program that makes use of Matlab's trapz (single integral). It has the benefit that it can be used to integrate over vectors x, y, and the function f(x,y), rather than on lower and upper limits. It can also be used with nonuniform spacing at the input vectors.

Example of nonuniform spacing:
x=[0,.1,.3,.45,.6,.8,.99,1]
y=[0,.05,.1,.2,.7,.57,.92,1]