Numerically integrate `sfit` object

## Syntax

```Q = quad2d(FO, a, b, c, d) [Q,ERRBND] = quad2d(...) [Q,ERRBND] = QUAD2D(FO,a,b,c,d,PARAM1,VAL1,PARAM2,VAL2,...) ```

## Description

`Q = quad2d(FO, a, b, c, d)` approximates the integral of the surface fit object `FO` over the planar region $a\le x\le b$ and $c\left(x\right)\le y\le d\left(x\right)$. The bounds `c` and `d` can each be a scalar, a function handle, or a curve fit (`cfit`) object.

`[Q,ERRBND] = quad2d(...)` also returns an approximate upper bound on the absolute error, `ERRBND`.

`[Q,ERRBND] = QUAD2D(FO,a,b,c,d,PARAM1,VAL1,PARAM2,VAL2,...)` performs the integration with specified values of optional parameters.

See the MATLAB® function `quad2d` for details of the upper bound and the optional parameters.