Sampling calculation documentation

Add a description of the sampling calculation in `Camera::Integrate()`.

``` c++
sample = (sample * ray_width * (1000 * 100 * 6378.1) / (4 / M_PI)) / 10000;
```

The calculation is described in detail in [SMILE's Red Book p23](https://sci.esa.int/documents/35028/36141/1567260374869-SMILE_RedBook_ESA_SCI_2018_1.pdf).